fix(cli): resolve environment loading and auth validation issues in ACP mode (#18025)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

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

@@ -11,8 +11,65 @@ import {
   toFriendlyError,
   BadRequestError,
   ForbiddenError,
+  getErrorMessage,
 } from './errors.js';
 
+describe('getErrorMessage', () => {
+  it('should return plain error message', () => {
+    expect(getErrorMessage(new Error('plain error'))).toBe('plain error');
+  });
+
+  it('should parse simple JSON error response', () => {
+    const json = JSON.stringify({ error: { message: 'json error' } });
+    expect(getErrorMessage(new Error(json))).toBe('json error');
+  });
+
+  it('should parse double-encoded JSON error response', () => {
+    const innerJson = JSON.stringify({ error: { message: 'nested error' } });
+    const outerJson = JSON.stringify({ error: { message: innerJson } });
+    expect(getErrorMessage(new Error(outerJson))).toBe('nested error');
+  });
+
+  it('should parse array-style JSON error response', () => {
+    const json = JSON.stringify([{ error: { message: 'array error' } }]);
+    expect(getErrorMessage(new Error(json))).toBe('array error');
+  });
+
+  it('should parse JSON with top-level message field', () => {
+    const json = JSON.stringify({ message: 'top-level message' });
+    expect(getErrorMessage(new Error(json))).toBe('top-level message');
+  });
+
+  it('should handle JSON with trailing newline', () => {
+    const json = JSON.stringify({ error: { message: 'newline error' } }) + '\n';
+    expect(getErrorMessage(new Error(json))).toBe('newline error');
+  });
+
+  it('should return original message if JSON parsing fails', () => {
+    const invalidJson = '{ not-json }';
+    expect(getErrorMessage(new Error(invalidJson))).toBe(invalidJson);
+  });
+
+  it('should handle non-Error inputs', () => {
+    expect(getErrorMessage('string error')).toBe('string error');
+    expect(getErrorMessage(123)).toBe('123');
+  });
+
+  it('should handle structured HTTP errors via toFriendlyError', () => {
+    const error = {
+      response: {
+        data: {
+          error: {
+            code: 400,
+            message: 'Bad Request Message',
+          },
+        },
+      },
+    };
+    expect(getErrorMessage(error)).toBe('Bad Request Message');
+  });
+});
+
 describe('isAuthenticationError', () => {
   it('should detect error with code: 401 property (MCP SDK style)', () => {
     const error = { code: 401, message: 'Unauthorized' };