fix(handlers): truncate manifest.json at last '}' before JSON parsing

The repo-root manifest.json has a trailing JSON5-style // comment
appended by the Integration Tester CI trigger (// Triggered by
Integration Tester at 2026-05-10T08:52Z).  Go's encoding/json throws
"invalid character '/' after top-level value" on the '/', causing
loadRuntimesFromManifest to fall back to the fallback allowlist at
runtime registry init.  In E2E Chat runs (MOLECULE_ENV=development),
the platform server starts from workspace-server/ and resolves
../manifest.json — the file with the trailing comment — so the echo
runtime is never registered and all round-trip Chat tests fail.

Fix: bytes.LastIndex(data, []byte("}")) + slice to that position
before json.Unmarshal, so any trailing comment or garbage after the
closing brace is discarded silently.

Two regression tests added:
- TestLoadRuntimesFromManifest_StripsJSON5TrailingComment:
  real manifest content + "// Triggered by ..." after }
- TestLoadRuntimesFromManifest_ExtraDataAfterClosingBrace:
  valid JSON + arbitrary trailing text

Closes #1480

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workspace-server/internal/handlers/runtime_registry.go

@@ -25,6 +25,7 @@ package handlers
 // to the pre-refactor hardcoded list so nothing regresses.
 
 import (
+	"bytes"
 	"encoding/json"
 	"log"
 	"os"
@@ -101,6 +102,13 @@ func loadRuntimesFromManifest(path string) (map[string]struct{}, error) {
 	if err != nil {
 		return nil, err
 	}
+	// Truncate at the last '}' to drop trailing JSON5-style // comments
+	// (e.g. "// Triggered by Integration Tester at 2026-05-10T08:52Z")
+	// that would otherwise cause Go's json.Unmarshal to fail with
+	// "invalid character '/' after top-level value".
+	if last := bytes.LastIndex(data, []byte("}")); last != -1 {
+		data = data[:last+1]
+	}
 	var m manifestFile
 	if err := json.Unmarshal(data, &m); err != nil {
 		return nil, err

workspace-server/internal/handlers/runtime_registry_test.go

@@ -83,6 +83,53 @@ func TestLoadRuntimesFromManifest_MalformedJSON(t *testing.T) {
 	}
 }
 
+func TestLoadRuntimesFromManifest_StripsJSON5TrailingComment(t *testing.T) {
+	// Regression: manifest.json has a trailing JSON5-style // comment appended
+	// by the Integration Tester (// Triggered by ...). Go's encoding/json
+	// throws "invalid character '/' after top-level value" on the '/'.
+	// loadRuntimesFromManifest must truncate at the last '}' before parsing
+	// so the comment is ignored and the valid JSON body is processed.
+	dir := t.TempDir()
+	path := filepath.Join(dir, "manifest.json")
+	content := `{
+		"workspace_templates": [
+			{"name": "claude-code-default", "repo": "org/t-cc"},
+			{"name": "langgraph", "repo": "org/t-lg"}
+		]
+	}
+	// Triggered by Integration Tester at 2026-05-10T08:52Z`
+	_ = os.WriteFile(path, []byte(content), 0600)
+
+	got, err := loadRuntimesFromManifest(path)
+	if err != nil {
+		t.Fatalf("expected successful parse despite trailing comment, got: %v", err)
+	}
+	for _, must := range []string{"claude-code", "langgraph", "external", "kimi", "kimi-cli"} {
+		if _, ok := got[must]; !ok {
+			t.Errorf("expected runtime %q in set after JSON5 comment strip: %v", must, keys(got))
+		}
+	}
+}
+
+func TestLoadRuntimesFromManifest_ExtraDataAfterClosingBrace(t *testing.T) {
+	// A file with trailing garbage after the closing '}' (not a // comment)
+	// should also be tolerated — the parser truncates at last '}';
+	// anything after is discarded.
+	dir := t.TempDir()
+	path := filepath.Join(dir, "manifest.json")
+	content := `{"workspace_templates":[{"name":"hermes","repo":"org/t"}]}
+extra data that should be ignored`
+	_ = os.WriteFile(path, []byte(content), 0600)
+
+	got, err := loadRuntimesFromManifest(path)
+	if err != nil {
+		t.Fatalf("expected parse with trailing data: %v", err)
+	}
+	if _, ok := got["hermes"]; !ok {
+		t.Errorf("expected hermes runtime after truncation: %v", keys(got))
+	}
+}
+
 // TestRealManifestParses — sanity check against the actual
 // monorepo manifest.json so a future schema change to that file
 // (e.g. workspace_templates → workspace_runtime_templates) surfaces