test: add isServerHealthy + startup error log tests

Tests for the new cross-platform health check helper (isServerHealthy)
that replaces PID-based liveness checks in all polling loops. Covers
healthy, unhealthy, unreachable, and error response cases.

Also tests the startup error log write/read format used on Windows.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

browse/test/config.test.ts

@@ -248,3 +248,69 @@ describe('version mismatch detection', () => {
     expect(shouldRestart).toBe(false);
   });
 });
+
+describe('isServerHealthy', () => {
+  const { isServerHealthy } = require('../src/cli');
+  const http = require('http');
+
+  test('returns true for a healthy server', async () => {
+    const server = http.createServer((_req: any, res: any) => {
+      res.writeHead(200, { 'Content-Type': 'application/json' });
+      res.end(JSON.stringify({ status: 'healthy' }));
+    });
+    await new Promise<void>(resolve => server.listen(0, resolve));
+    const port = server.address().port;
+    try {
+      expect(await isServerHealthy(port)).toBe(true);
+    } finally {
+      server.close();
+    }
+  });
+
+  test('returns false for an unhealthy server', async () => {
+    const server = http.createServer((_req: any, res: any) => {
+      res.writeHead(200, { 'Content-Type': 'application/json' });
+      res.end(JSON.stringify({ status: 'unhealthy' }));
+    });
+    await new Promise<void>(resolve => server.listen(0, resolve));
+    const port = server.address().port;
+    try {
+      expect(await isServerHealthy(port)).toBe(false);
+    } finally {
+      server.close();
+    }
+  });
+
+  test('returns false when server is not running', async () => {
+    // Use a port that's almost certainly not in use
+    expect(await isServerHealthy(59999)).toBe(false);
+  });
+
+  test('returns false on non-200 response', async () => {
+    const server = http.createServer((_req: any, res: any) => {
+      res.writeHead(500);
+      res.end('Internal Server Error');
+    });
+    await new Promise<void>(resolve => server.listen(0, resolve));
+    const port = server.address().port;
+    try {
+      expect(await isServerHealthy(port)).toBe(false);
+    } finally {
+      server.close();
+    }
+  });
+});
+
+describe('startup error log', () => {
+  test('write and read error log', () => {
+    const tmpDir = path.join(os.tmpdir(), `browse-error-log-test-${Date.now()}`);
+    fs.mkdirSync(tmpDir, { recursive: true });
+    const errorLogPath = path.join(tmpDir, 'browse-startup-error.log');
+    const errorMsg = 'Cannot find module playwright';
+    fs.writeFileSync(errorLogPath, `2026-03-23T00:00:00.000Z ${errorMsg}\n`);
+    const content = fs.readFileSync(errorLogPath, 'utf-8').trim();
+    expect(content).toContain(errorMsg);
+    expect(content).toMatch(/^\d{4}-\d{2}-\d{2}T/); // ISO timestamp prefix
+    fs.rmSync(tmpDir, { recursive: true, force: true });
+  });
+});