test(graph): pin graceful degradation for Dart extension types

The vendored @ast-grep/lang-dart 0.0.7 grammar predates Dart 3.3
extension types and parses them to ERROR nodes; the kinds a newer
tree-sitter-dart exposes (extension_type_declaration) do not exist in
this version, so matching on them would be unreachable code. Document
the limitation at the walk site and lock in the contract: no throw, no
bogus symbols from the ERROR region, and the rest of the file still
extracts normally.

Author: Giancarlo Erra

src/services/graph-symbols.ts

@@ -399,6 +399,11 @@ function extractFromDart(
   };
 
   // ── Top-level declarations (ordered walk so signature/body pairs line up) ──
+  // Dart 3.3 `extension type` is NOT handled: the vendored grammar
+  // (@ast-grep/lang-dart 0.0.7) predates the syntax and parses it to ERROR
+  // nodes (no extension_type_declaration kind exists), so such declarations
+  // degrade to "not extracted" while the rest of the file extracts normally.
+  // Revisit when the upstream grammar adds the kind.
   const topLevel = kidsOf(root);
   for (let i = 0; i < topLevel.length; i++) {
     const node = topLevel[i];

tests/unit/graph-symbols.test.ts

@@ -575,6 +575,26 @@ void main() {
       expect(callees).toContain("helper");
     });
 
+    it("degrades gracefully on Dart 3.3 extension types (unsupported by grammar 0.0.7)", () => {
+      // The vendored tree-sitter grammar predates `extension type` and parses
+      // it to ERROR nodes. The contract: no throw, no bogus symbols from the
+      // ERROR region, and the rest of the file still extracts normally.
+      const src = `
+extension type Meters(int value) {
+  int get inKm => value ~/ 1000;
+}
+
+class Real {
+  void work() {}
+}
+`;
+      const out = extractSymbolsAndCalls(src, "dart" as unknown as Lang, ".dart", "lib/m.dart");
+      expect(out.symbols.some((s) => s.qualifiedName === "Real" && s.kind === "class")).toBe(true);
+      expect(out.symbols.some((s) => s.qualifiedName === "Real.work" && s.kind === "method")).toBe(true);
+      // The unsupported declaration produces no symbol named Meters
+      expect(out.symbols.some((s) => s.name === "Meters")).toBe(false);
+    });
+
     it("detects main() so Dart apps get a conventional entry point", () => {
       const src = `
 void main() {