feat(core,cli): harden headless mode detection and align mocks

- Update isHeadlessMode in packages/core to check both stdin.isTTY and stdout.isTTY.
- Synchronize isHeadlessMode mock in packages/cli tests and add global TTY stubs to ensure consistent test environments.
- Add isMounted guard to useFolderTrust hook to prevent state updates on unmounted components in headless mode.
- Expand unit tests in packages/core to cover new TTY check combinations and edge cases.
- Stabilize flaky MCP initialization test in packages/core/src/config/config.test.ts by using a deterministic promise.
- Address review findings regarding environment detection consistency and CI indicator checks.

packages/core/src/utils/headless.test.ts

@@ -0,0 +1,146 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { isHeadlessMode } from './headless.js';
+import process from 'node:process';
+
+describe('isHeadlessMode', () => {
+  const originalStdoutIsTTY = process.stdout.isTTY;
+  const originalStdinIsTTY = process.stdin.isTTY;
+
+  beforeEach(() => {
+    vi.stubEnv('CI', '');
+    vi.stubEnv('GITHUB_ACTIONS', '');
+    // We can't easily stub process.stdout.isTTY with vi.stubEnv
+    // So we'll use Object.defineProperty
+    Object.defineProperty(process.stdout, 'isTTY', {
+      value: true,
+      configurable: true,
+    });
+    Object.defineProperty(process.stdin, 'isTTY', {
+      value: true,
+      configurable: true,
+    });
+  });
+
+  afterEach(() => {
+    vi.unstubAllEnvs();
+    Object.defineProperty(process.stdout, 'isTTY', {
+      value: originalStdoutIsTTY,
+      configurable: true,
+    });
+    Object.defineProperty(process.stdin, 'isTTY', {
+      value: originalStdinIsTTY,
+      configurable: true,
+    });
+    vi.restoreAllMocks();
+  });
+
+  it('should return false in a normal TTY environment', () => {
+    expect(isHeadlessMode()).toBe(false);
+  });
+
+  it('should return true if CI environment variable is "true"', () => {
+    vi.stubEnv('CI', 'true');
+    expect(isHeadlessMode()).toBe(true);
+  });
+
+  it('should return true if GITHUB_ACTIONS environment variable is "true"', () => {
+    vi.stubEnv('GITHUB_ACTIONS', 'true');
+    expect(isHeadlessMode()).toBe(true);
+  });
+
+  it('should return true if stdout is not a TTY', () => {
+    Object.defineProperty(process.stdout, 'isTTY', {
+      value: false,
+      configurable: true,
+    });
+    expect(isHeadlessMode()).toBe(true);
+  });
+
+  it('should return true if stdin is not a TTY', () => {
+    Object.defineProperty(process.stdin, 'isTTY', {
+      value: false,
+      configurable: true,
+    });
+    expect(isHeadlessMode()).toBe(true);
+  });
+
+  it('should return true if stdin is a TTY but stdout is not', () => {
+    Object.defineProperty(process.stdin, 'isTTY', {
+      value: true,
+      configurable: true,
+    });
+    Object.defineProperty(process.stdout, 'isTTY', {
+      value: false,
+      configurable: true,
+    });
+    expect(isHeadlessMode()).toBe(true);
+  });
+
+  it('should return true if stdout is a TTY but stdin is not', () => {
+    Object.defineProperty(process.stdin, 'isTTY', {
+      value: false,
+      configurable: true,
+    });
+    Object.defineProperty(process.stdout, 'isTTY', {
+      value: true,
+      configurable: true,
+    });
+    expect(isHeadlessMode()).toBe(true);
+  });
+
+  it('should return true if a prompt option is provided', () => {
+    expect(isHeadlessMode({ prompt: 'test prompt' })).toBe(true);
+    expect(isHeadlessMode({ prompt: true })).toBe(true);
+  });
+
+  it('should return false if query is provided but it is still a TTY', () => {
+    // Note: per current logic, query alone doesn't force headless if TTY
+    // This matches the existing behavior in packages/cli/src/config/config.ts
+    expect(isHeadlessMode({ query: 'test query' })).toBe(false);
+  });
+
+  it('should handle undefined process.stdout gracefully', () => {
+    const originalStdout = process.stdout;
+    // @ts-expect-error - testing edge case
+    delete process.stdout;
+
+    try {
+      expect(isHeadlessMode()).toBe(false);
+    } finally {
+      Object.defineProperty(process, 'stdout', {
+        value: originalStdout,
+        configurable: true,
+      });
+    }
+  });
+
+  it('should handle undefined process.stdin gracefully', () => {
+    const originalStdin = process.stdin;
+    // @ts-expect-error - testing edge case
+    delete process.stdin;
+
+    try {
+      expect(isHeadlessMode()).toBe(false);
+    } finally {
+      Object.defineProperty(process, 'stdin', {
+        value: originalStdin,
+        configurable: true,
+      });
+    }
+  });
+
+  it('should return true if multiple headless indicators are set', () => {
+    vi.stubEnv('CI', 'true');
+    Object.defineProperty(process.stdout, 'isTTY', {
+      value: false,
+      configurable: true,
+    });
+    expect(isHeadlessMode({ prompt: true })).toBe(true);
+  });
+});