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/tests/unit/core/api/stream-mode.test.ts

@@ -0,0 +1,40 @@
+import { expect, test } from "vitest";
+
+import { sanitizeRunStreamOptions } from "@/core/api/stream-mode";
+
+test("drops unsupported stream modes from array payloads", () => {
+  const sanitized = sanitizeRunStreamOptions({
+    streamMode: [
+      "values",
+      "messages-tuple",
+      "custom",
+      "updates",
+      "events",
+      "tools",
+    ],
+  });
+
+  expect(sanitized.streamMode).toEqual([
+    "values",
+    "messages-tuple",
+    "custom",
+    "updates",
+    "events",
+  ]);
+});
+
+test("drops unsupported stream modes from scalar payloads", () => {
+  const sanitized = sanitizeRunStreamOptions({
+    streamMode: "tools",
+  });
+
+  expect(sanitized.streamMode).toBeUndefined();
+});
+
+test("keeps payloads without streamMode untouched", () => {
+  const options = {
+    streamSubgraphs: true,
+  };
+
+  expect(sanitizeRunStreamOptions(options)).toBe(options);
+});