Fix debug assertion violation for document highlights on export = in merged namespace (#3818)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>

Author: Copilot

internal/fourslash/tests/documentHighlightsExportEqualsInMergedNamespace_test.go

@@ -0,0 +1,22 @@
+package fourslash_test
+
+import (
+	"testing"
+
+	"github.com/microsoft/typescript-go/internal/fourslash"
+	"github.com/microsoft/typescript-go/internal/testutil"
+)
+
+func TestDocumentHighlightsExportEqualsInMergedNamespace(t *testing.T) {
+	t.Parallel()
+	defer testutil.RecoverAndFail(t, "Panic on fourslash test")
+	const content = `
+class C {}
+namespace C {
+    /*marker*/export = C;
+}
+`
+	f, done := fourslash.NewFourslash(t, nil /*capabilities*/, content)
+	defer done()
+	f.VerifyBaselineDocumentHighlights(t, nil /*preferences*/, "marker")
+}

internal/ls/findallreferences.go

@@ -1373,8 +1373,8 @@ func (l *LanguageService) getReferencedSymbolsForModule(ctx context.Context, pro
 					references = append(references, newNodeEntry(decl.AsModuleDeclaration().Name()))
 				}
 			default:
-				// This may be merged with something.
-				debug.Assert(symbol.Flags&ast.SymbolFlagsTransient != 0, "Expected a module symbol to be declared by a SourceFile or ModuleDeclaration.")
+				// This may be merged with something (e.g. a class merged with a namespace).
+				continue
 			}
 		}
 	}

testdata/baselines/reference/fourslash/documentHighlights/documentHighlightsExportEqualsInMergedNamespace.baseline.jsonc

@@ -0,0 +1,8 @@
+// === documentHighlights ===
+// === /documentHighlightsExportEqualsInMergedNamespace.ts ===
+// 
+// class C {}
+// namespace [|C|] {
+//     /*HIGHLIGHTS*/[|export|] = C;
+// }
+//