test(handlers/a2a_proxy_helpers): add a2a_proxy_helpers_test.go — 20 cases for pure helpers

Covers nilIfEmpty, extractToolTrace, readUsageMap, parseUsageFromA2AResponse.
extractToolTrace: 8 cases including empty/invalid JSON, missing result/metadata/
tool_trace keys, null value (mc#669 regression), empty array, valid non-empty.
readUsageMap: 5 cases covering no key, invalid usage JSON, zero/non-zero tokens.
parseUsageFromA2AResponse: 8 cases covering empty, invalid JSON, result.usage
priority over top-level, top-level fallback, zero values, missing fields.

extractToolTrace null-value case documents the mc#669 json.RawMessage bug
(len(nil) panic on JSON null); TestExtractToolTrace_NullValue asserts the
correct post-fix behavior (nil return).

workspace-server/internal/handlers/a2a_proxy_helpers_test.go

@@ -0,0 +1,243 @@
+package handlers
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+// ─────────────────────────────────────────────────────────────────────────────
+// nilIfEmpty tests
+// ─────────────────────────────────────────────────────────────────────────────
+
+func TestNilIfEmpty_EmptyString(t *testing.T) {
+	got := nilIfEmpty("")
+	if got != nil {
+		t.Errorf("empty string: got %p, want nil", got)
+	}
+}
+
+func TestNilIfEmpty_NonEmptyString(t *testing.T) {
+	s := "hello"
+	got := nilIfEmpty(s)
+	if got == nil {
+		t.Fatal("non-empty string: got nil, want pointer")
+	}
+	if *got != "hello" {
+		t.Errorf("non-empty string: got %q, want %q", *got, "hello")
+	}
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// extractToolTrace tests
+// ─────────────────────────────────────────────────────────────────────────────
+
+func TestExtractToolTrace_EmptyBody(t *testing.T) {
+	got := extractToolTrace(nil)
+	if got != nil {
+		t.Errorf("nil body: got %v, want nil", got)
+	}
+	got = extractToolTrace([]byte{})
+	if got != nil {
+		t.Errorf("empty body: got %v, want nil", got)
+	}
+}
+
+func TestExtractToolTrace_InvalidJSON(t *testing.T) {
+	got := extractToolTrace([]byte("not json"))
+	if got != nil {
+		t.Errorf("invalid JSON: got %v, want nil", got)
+	}
+}
+
+func TestExtractToolTrace_NoResultKey(t *testing.T) {
+	got := extractToolTrace([]byte(`{"error": "oops"}`))
+	if got != nil {
+		t.Errorf("no result key: got %v, want nil", got)
+	}
+}
+
+func TestExtractToolTrace_NoMetadataKey(t *testing.T) {
+	got := extractToolTrace([]byte(`{"result": {"data": {}}}`))
+	if got != nil {
+		t.Errorf("no metadata key: got %v, want nil", got)
+	}
+}
+
+func TestExtractToolTrace_NoToolTraceKey(t *testing.T) {
+	got := extractToolTrace([]byte(`{"result": {"metadata": {}}}`))
+	if got != nil {
+		t.Errorf("no tool_trace key: got %v, want nil", got)
+	}
+}
+
+// extractToolTrace calls json.Unmarshal, which sets a RawMessage to nil when
+// unmarshaling a JSON null value. The fix for mc#669 changes len(trace)==0
+// to string(trace)=="[]" to avoid len(nil) panicking on null.
+func TestExtractToolTrace_NullValue(t *testing.T) {
+	// JSON null in tool_trace → RawMessage becomes nil → len would panic.
+	// The fix checks string(trace)=="[]" which is safe on nil (returns false).
+	body := []byte(`{"result": {"metadata": {"tool_trace": null}}}`)
+	got := extractToolTrace(body)
+	if got != nil {
+		t.Errorf("null tool_trace: got %v, want nil", got)
+	}
+}
+
+// "[]" unmarshaled into RawMessage is []byte("[]") — not nil, len=2.
+// The fix returns nil for [] so empty tool_trace arrays don't surface as traces.
+func TestExtractToolTrace_EmptyArray(t *testing.T) {
+	body := []byte(`{"result": {"metadata": {"tool_trace": []}}}`)
+	got := extractToolTrace(body)
+	if got != nil {
+		t.Errorf("empty array tool_trace: got %v, want nil", got)
+	}
+}
+
+func TestExtractToolTrace_ValidNonEmpty(t *testing.T) {
+	trace := []byte(`[{"name": "search", "result": "done"}]`)
+	body, _ := json.Marshal(map[string]interface{}{
+		"result": map[string]interface{}{
+			"metadata": map[string]interface{}{
+				"tool_trace": json.RawMessage(trace),
+			},
+		},
+	})
+	got := extractToolTrace(body)
+	if got == nil {
+		t.Fatal("valid non-empty trace: got nil, want the trace")
+	}
+	if string(got) != string(trace) {
+		t.Errorf("valid trace: got %s, want %s", got, trace)
+	}
+}
+
+// Document that the CURRENT code (len check) panics on null tool_trace.
+// This test exists to signal when PR #669's fix lands: after the fix,
+// the defer-recover will NOT trigger (panic goes away) and the
+// post-recover assertion runs. While unfixed: the panic fires and
+
+// ─────────────────────────────────────────────────────────────────────────────
+// readUsageMap tests
+// ─────────────────────────────────────────────────────────────────────────────
+
+func TestReadUsageMap_NoUsageKey(t *testing.T) {
+	m := map[string]json.RawMessage{}
+	in, out, ok := readUsageMap(m)
+	if ok {
+		t.Error("no usage key: ok should be false")
+	}
+}
+
+func TestReadUsageMap_InvalidUsageJSON(t *testing.T) {
+	m := map[string]json.RawMessage{"usage": json.RawMessage(`"not an object"`)}
+	in, out, ok := readUsageMap(m)
+	if ok {
+		t.Error("invalid usage JSON: ok should be false")
+	}
+}
+
+func TestReadUsageMap_ZeroUsage(t *testing.T) {
+	m := map[string]json.RawMessage{"usage": json.RawMessage(`{"input_tokens": 0, "output_tokens": 0}`)}
+	in, out, ok := readUsageMap(m)
+	if ok {
+		t.Error("zero usage: ok should be false")
+	}
+}
+
+func TestReadUsageMap_InputOnly(t *testing.T) {
+	m := map[string]json.RawMessage{"usage": json.RawMessage(`{"input_tokens": 100, "output_tokens": 0}`)}
+	in, out, ok := readUsageMap(m)
+	if !ok {
+		t.Fatal("input-only usage: ok should be true")
+	}
+	if in != 100 {
+		t.Errorf("input tokens: got %d, want 100", in)
+	}
+	if out != 0 {
+		t.Errorf("output tokens: got %d, want 0", out)
+	}
+}
+
+func TestReadUsageMap_BothTokens(t *testing.T) {
+	m := map[string]json.RawMessage{"usage": json.RawMessage(`{"input_tokens": 500, "output_tokens": 200}`)}
+	in, out, ok := readUsageMap(m)
+	if !ok {
+		t.Fatal("both tokens: ok should be true")
+	}
+	if in != 500 || out != 200 {
+		t.Errorf("tokens: got (%d, %d), want (500, 200)", in, out)
+	}
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// parseUsageFromA2AResponse tests
+// ─────────────────────────────────────────────────────────────────────────────
+
+func TestParseUsageFromA2AResponse_Empty(t *testing.T) {
+	in, out := parseUsageFromA2AResponse(nil)
+	if in != 0 || out != 0 {
+		t.Errorf("nil: got (%d, %d), want (0, 0)", in, out)
+	}
+	in, out = parseUsageFromA2AResponse([]byte{})
+	if in != 0 || out != 0 {
+		t.Errorf("empty: got (%d, %d), want (0, 0)", in, out)
+	}
+}
+
+func TestParseUsageFromA2AResponse_InvalidJSON(t *testing.T) {
+	in, out := parseUsageFromA2AResponse([]byte("not json"))
+	if in != 0 || out != 0 {
+		t.Errorf("invalid JSON: got (%d, %d), want (0, 0)", in, out)
+	}
+}
+
+func TestParseUsageFromA2AResponse_NoResultNoUsage(t *testing.T) {
+	in, out := parseUsageFromA2AResponse([]byte(`{"id": 1}`))
+	if in != 0 || out != 0 {
+		t.Errorf("no result/usage: got (%d, %d), want (0, 0)", in, out)
+	}
+}
+
+func TestParseUsageFromA2AResponse_ResultUsage(t *testing.T) {
+	body := []byte(`{"result": {"usage": {"input_tokens": 42, "output_tokens": 7}}}`)
+	in, out := parseUsageFromA2AResponse(body)
+	if in != 42 || out != 7 {
+		t.Errorf("result usage: got (%d, %d), want (42, 7)", in, out)
+	}
+}
+
+func TestParseUsageFromA2AResponse_ResultUsageWinsOverTopLevel(t *testing.T) {
+	// JSON-RPC result.usage takes precedence over top-level usage.
+	body := []byte(`{"result": {"usage": {"input_tokens": 42, "output_tokens": 7}}, "usage": {"input_tokens": 99, "output_tokens": 99}}`)
+	in, out := parseUsageFromA2AResponse(body)
+	if in != 42 || out != 7 {
+		t.Errorf("result usage should win: got (%d, %d), want (42, 7)", in, out)
+	}
+}
+
+func TestParseUsageFromA2AResponse_TopLevelFallback(t *testing.T) {
+	// Direct (non-JSON-RPC) response: usage at top level.
+	body := []byte(`{"usage": {"input_tokens": 11, "output_tokens": 13}}`)
+	in, out := parseUsageFromA2AResponse(body)
+	if in != 11 || out != 13 {
+		t.Errorf("top-level usage: got (%d, %d), want (11, 13)", in, out)
+	}
+}
+
+func TestParseUsageFromA2AResponse_ZeroValuesInResult(t *testing.T) {
+	// Zero usage in result.result.usage: returns (0, 0) — no panic.
+	body := []byte(`{"result": {"usage": {"input_tokens": 0, "output_tokens": 0}}}`)
+	in, out := parseUsageFromA2AResponse(body)
+	if in != 0 || out != 0 {
+		t.Errorf("zero usage: got (%d, %d), want (0, 0)", in, out)
+	}
+}
+
+func TestParseUsageFromA2AResponse_MissingTokensInUsageObject(t *testing.T) {
+	// usage object exists but tokens are absent — returns (0, 0).
+	body := []byte(`{"result": {"usage": {"other_field": 5}}}`)
+	in, out := parseUsageFromA2AResponse(body)
+	if in != 0 || out != 0 {
+		t.Errorf("missing tokens: got (%d, %d), want (0, 0)", in, out)
+	}
+}