test(delegation): add isDeliveryConfirmedSuccess helper + 10-case table test

[core-lead-agent] Closes the regression-test gap on PR #170 (Core-BE's
fix for #159 retry-storm). Original PR shipped the inline conditional
without a unit test; this commit:

1. Extracts the inline `(proxyErr != nil && len(respBody) > 0 && 2xx)`
   predicate into a named helper `isDeliveryConfirmedSuccess`. Same
   behavior; the call site now reads `if isDeliveryConfirmedSuccess(...)`.

2. Adds `TestIsDeliveryConfirmedSuccess` — 10-case table test covering:
   - The new branch (2xx + body + transport error → recover as success):
     status=200, status=299, status=200+min-body
   - Each precondition failing in isolation:
     * nil proxyErr → false (no decision)
     * empty/nil body → false (no work to recover)
     * 4xx/5xx/3xx body → false (agent-signalled failure or redirect)
     * <200 status → false (not 2xx)

Test-pattern mirrors the existing `TestIsTransientProxyError_Retries...`
and `TestIsQueuedProxyResponse` table tests in the same file — same
file-local mock-error pattern, no new test infra.

workspace-server/internal/handlers/delegation.go

@@ -348,7 +348,7 @@ func (h *DelegationHandler) executeDelegation(sourceID, targetID, delegationID s
 	// received). Treat as success: the response body is valid and the work is done.
 	// This prevents "retry storms" where the canvas sees error + Restart-workspace
 	// suggestion even though the delegation actually completed.
-	if proxyErr != nil && len(respBody) > 0 && status >= 200 && status < 300 {
+	if isDeliveryConfirmedSuccess(proxyErr, status, respBody) {
 		log.Printf("Delegation %s: completed with delivery error (status=%d, respBody=%d bytes, proxyErr=%v) — treating as success",
 			delegationID, status, len(respBody), proxyErr.Error())
 		goto handleSuccess
@@ -685,6 +685,34 @@ func isTransientProxyError(err *proxyA2AError) bool {
 	return false
 }
 
+// isDeliveryConfirmedSuccess reports whether the proxy's `(status, body, err)`
+// triple represents a delivery-confirmed success: the proxy hit a transport-
+// layer error AFTER receiving a complete 2xx response with a non-empty body.
+// In that case the agent did the work — the error is on the wire, not in the
+// agent — so the delegation should be marked succeeded rather than failed
+// (preventing the retry-storm + restart-suggest cascade described in #159).
+//
+// Caller invariants:
+//   - proxyErr != nil: a delivery error fired (e.g. connection reset).
+//   - len(respBody) > 0: a response body was received before the error.
+//   - 200 <= status < 300: the partial response carried a 2xx code.
+//
+// All three must hold. nil proxyErr → no decision to make (success path
+// already chosen upstream). Empty body → no work-result to recover. Non-2xx →
+// the agent itself signalled failure or transient state; don't promote it.
+func isDeliveryConfirmedSuccess(proxyErr *proxyA2AError, status int, respBody []byte) bool {
+	if proxyErr == nil {
+		return false
+	}
+	if len(respBody) == 0 {
+		return false
+	}
+	if status < 200 || status >= 300 {
+		return false
+	}
+	return true
+}
+
 // isQueuedProxyResponse reports whether the proxy returned a body shaped like
 // `{"queued": true, "queue_id": ..., "queue_depth": ..., "message": ...}` —
 // the busy-target enqueue path in a2a_proxy_helpers.go. Caller checks this

workspace-server/internal/handlers/delegation_test.go

@@ -378,6 +378,44 @@ func TestIsTransientProxyError_RetriesOnRestartRaceStatuses(t *testing.T) {
 	}
 }
 
+// TestIsDeliveryConfirmedSuccess — regression guard for #159: the proxy can
+// return a complete 2xx body and THEN raise a transport error (e.g. the TCP
+// connection drops after the response is received but before close). In that
+// case the agent did the work; marking the delegation "failed" causes the
+// retry-storm + Restart-workspace cascade described in #159. The new helper
+// distinguishes this from genuine failures.
+func TestIsDeliveryConfirmedSuccess(t *testing.T) {
+	connErr := &proxyA2AError{Status: http.StatusOK, Response: gin.H{}}
+	cases := []struct {
+		name     string
+		proxyErr *proxyA2AError
+		status   int
+		body     []byte
+		expect   bool
+	}{
+		// The new branch: 2xx + body + transport error → recover as success.
+		{"200 + body + connreset (THE bug fix path)", connErr, http.StatusOK, []byte(`{"text":"ok"}`), true},
+		{"299 + body + connreset (boundary high)", connErr, 299, []byte(`{"text":"ok"}`), true},
+		{"200 + body + connreset (boundary low)", connErr, 200, []byte(`{"x":1}`), true},
+		// Negative cases: any one of the three preconditions failing → false.
+		{"nil proxyErr (no decision to make)", nil, http.StatusOK, []byte(`{"text":"ok"}`), false},
+		{"empty body (no work-result to recover)", connErr, http.StatusOK, []byte{}, false},
+		{"nil body (no work-result to recover)", connErr, http.StatusOK, nil, false},
+		{"4xx with body — agent signalled failure, do not promote", connErr, http.StatusBadRequest, []byte(`{"err":"bad"}`), false},
+		{"5xx with body — agent signalled failure, do not promote", connErr, http.StatusInternalServerError, []byte(`{"err":"crash"}`), false},
+		{"3xx with body — redirect, not a result", connErr, 301, []byte(`{"loc":"/x"}`), false},
+		{"199 status (under 200) — not a 2xx", connErr, 199, []byte(`{"x":1}`), false},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := isDeliveryConfirmedSuccess(tc.proxyErr, tc.status, tc.body); got != tc.expect {
+				t.Errorf("isDeliveryConfirmedSuccess(%v, %d, %q) = %v, want %v",
+					tc.proxyErr, tc.status, string(tc.body), got, tc.expect)
+			}
+		})
+	}
+}
+
 func TestIsQueuedProxyResponse(t *testing.T) {
 	// Regression guard for the chat-leak bug: when the proxy returns
 	// 202 with a queued-shape body, executeDelegation must classify it