test(utils): add unit tests for parseJsonWithJson5Fallback (#98801)

* test(utils): add unit tests for parseJsonWithJson5Fallback

* test(utils): verify JSON.parse fast path with spy, per ClawSweeper review

- Add vi.spyOn to prove JSON.parse is called (not JSON5) for strict JSON
- Add spy to prove JSON5.parse is called on fallback path
- Run full src/utils test suite (263 tests passed)

src/utils/parse-json-compat.test.ts

@@ -0,0 +1,44 @@
+// Parse JSON compat tests cover strict-then-JSON5 fallback parsing.
+import JSON5 from "json5";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { parseJsonWithJson5Fallback } from "./parse-json-compat.js";
+
+describe("parseJsonWithJson5Fallback", () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("parses strict JSON via JSON.parse without invoking JSON5", () => {
+    const jsonParseSpy = vi.spyOn(JSON, "parse");
+    const json5ParseSpy = vi.spyOn(JSON5, "parse");
+
+    expect(parseJsonWithJson5Fallback('{"a":1}')).toEqual({ a: 1 });
+    expect(jsonParseSpy).toHaveBeenCalled();
+    expect(json5ParseSpy).not.toHaveBeenCalled();
+  });
+
+  it("falls back to JSON5 when JSON.parse throws", () => {
+    const json5ParseSpy = vi.spyOn(JSON5, "parse");
+
+    expect(parseJsonWithJson5Fallback('{"a":1,}')).toEqual({ a: 1 });
+    expect(json5ParseSpy).toHaveBeenCalled();
+  });
+
+  it("handles trailing commas via JSON5 fallback", () => {
+    expect(parseJsonWithJson5Fallback("[1,2,]")).toEqual([1, 2]);
+  });
+
+  it("handles JSON5 comments", () => {
+    expect(parseJsonWithJson5Fallback('{\n  // c\n  "a": 1\n}')).toEqual({
+      a: 1,
+    });
+  });
+
+  it("handles JSON5 single-quoted strings", () => {
+    expect(parseJsonWithJson5Fallback("{'a':1}")).toEqual({ a: 1 });
+  });
+
+  it("throws when both JSON and JSON5 parsing fail", () => {
+    expect(() => parseJsonWithJson5Fallback("{invalid")).toThrow();
+  });
+});