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/cli/src/config/config.test.ts

@@ -140,6 +140,14 @@ vi.mock('@google/gemini-cli-core', async () => {
       defaultDecision: ServerConfig.PolicyDecision.ASK_USER,
       approvalMode: ServerConfig.ApprovalMode.DEFAULT,
     })),
+    isHeadlessMode: vi.fn(
+      (opts) =>
+        !!opts?.prompt ||
+        process.env['CI'] === 'true' ||
+        process.env['GITHUB_ACTIONS'] === 'true' ||
+        (!!process.stdin && !process.stdin.isTTY) ||
+        (!!process.stdout && !process.stdout.isTTY),
+    ),
   };
 });
 
@@ -153,6 +161,8 @@ vi.mock('./extension-manager.js', () => {
 // Global setup to ensure clean environment for all tests in this file
 const originalArgv = process.argv;
 const originalGeminiModel = process.env['GEMINI_MODEL'];
+const originalStdoutIsTTY = process.stdout.isTTY;
+const originalStdinIsTTY = process.stdin.isTTY;
 
 beforeEach(() => {
   delete process.env['GEMINI_MODEL'];
@@ -161,6 +171,18 @@ beforeEach(() => {
   ExtensionManager.prototype.loadExtensions = vi
     .fn()
     .mockResolvedValue(undefined);
+
+  // Default to interactive mode for tests unless otherwise specified
+  Object.defineProperty(process.stdout, 'isTTY', {
+    value: true,
+    configurable: true,
+    writable: true,
+  });
+  Object.defineProperty(process.stdin, 'isTTY', {
+    value: true,
+    configurable: true,
+    writable: true,
+  });
 });
 
 afterEach(() => {
@@ -170,6 +192,16 @@ afterEach(() => {
   } else {
     delete process.env['GEMINI_MODEL'];
   }
+  Object.defineProperty(process.stdout, 'isTTY', {
+    value: originalStdoutIsTTY,
+    configurable: true,
+    writable: true,
+  });
+  Object.defineProperty(process.stdin, 'isTTY', {
+    value: originalStdinIsTTY,
+    configurable: true,
+    writable: true,
+  });
 });
 
 describe('parseArguments', () => {
@@ -248,6 +280,16 @@ describe('parseArguments', () => {
   });
 
   describe('positional arguments and @commands', () => {
+    beforeEach(() => {
+      // Default to headless mode for these tests as they mostly expect one-shot behavior
+      process.stdin.isTTY = false;
+      Object.defineProperty(process.stdout, 'isTTY', {
+        value: false,
+        configurable: true,
+        writable: true,
+      });
+    });
+
     it.each([
       {
         description:
@@ -378,8 +420,12 @@ describe('parseArguments', () => {
     );
 
     it('should include a startup message when converting positional query to interactive prompt', async () => {
-      const originalIsTTY = process.stdin.isTTY;
       process.stdin.isTTY = true;
+      Object.defineProperty(process.stdout, 'isTTY', {
+        value: true,
+        configurable: true,
+        writable: true,
+      });
       process.argv = ['node', 'script.js', 'hello'];
 
       try {
@@ -388,7 +434,7 @@ describe('parseArguments', () => {
           'Positional arguments now default to interactive mode. To run in non-interactive mode, use the --prompt (-p) flag.',
         );
       } finally {
-        process.stdin.isTTY = originalIsTTY;
+        // beforeEach handles resetting
       }
     });
   });
@@ -2573,6 +2619,16 @@ describe('Output format', () => {
 describe('parseArguments with positional prompt', () => {
   const originalArgv = process.argv;
 
+  beforeEach(() => {
+    // Default to headless mode for these tests as they mostly expect one-shot behavior
+    process.stdin.isTTY = false;
+    Object.defineProperty(process.stdout, 'isTTY', {
+      value: false,
+      configurable: true,
+      writable: true,
+    });
+  });
+
   afterEach(() => {
     process.argv = originalArgv;
   });