feat(frontend): set up Vitest frontend testing infrastructure with CI workflow (#2147)

* feat: set up Vitest frontend testing infrastructure with CI workflow

Migrate existing 4 frontend test files from Node.js native test runner
(node:test + node:assert/strict) to Vitest, reorganize test directory
structure under tests/unit/ mirroring src/ layout, and add a dedicated
CI workflow for frontend unit tests.

- Add vitest as devDependency, remove tsx
- Create vitest.config.ts with @/ path alias
- Migrate tests to Vitest API (test/expect/vi)
- Rename .mjs test files to .ts
- Move tests from src/ to tests/unit/ (mirrors src/ layout)
- Add frontend/Makefile `test` target
- Add .github/workflows/frontend-unit-tests.yml (parallel to backend)
- Update CONTRIBUTING.md, README.md, AGENTS.md, CLAUDE.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* style: fix the lint error

* style: fix the lint error

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

frontend/src/core/api/stream-mode.test.ts

@@ -1,43 +0,0 @@
-import assert from "node:assert/strict";
-import test from "node:test";
-
-const { sanitizeRunStreamOptions } = await import(
-  new URL("./stream-mode.ts", import.meta.url).href
-);
-
-void test("drops unsupported stream modes from array payloads", () => {
-  const sanitized = sanitizeRunStreamOptions({
-    streamMode: [
-      "values",
-      "messages-tuple",
-      "custom",
-      "updates",
-      "events",
-      "tools",
-    ],
-  });
-
-  assert.deepEqual(sanitized.streamMode, [
-    "values",
-    "messages-tuple",
-    "custom",
-    "updates",
-    "events",
-  ]);
-});
-
-void test("drops unsupported stream modes from scalar payloads", () => {
-  const sanitized = sanitizeRunStreamOptions({
-    streamMode: "tools",
-  });
-
-  assert.equal(sanitized.streamMode, undefined);
-});
-
-void test("keeps payloads without streamMode untouched", () => {
-  const options = {
-    streamSubgraphs: true,
-  };
-
-  assert.equal(sanitizeRunStreamOptions(options), options);
-});