fix(core): route personal OAuth users to stable models and fix test regressions

This PR resolves multiple issues:
1.  **Auth-Aware Model Routing**: Implements routing to stable models for users using personal OAuth (`oauth-personal`) to prevent 404/400 errors for unavailable preview models.
2.  **Config Class Stability**: Fixes a crash in `Config.getAuthType` by adding optional chaining for `contentGeneratorConfig`, which could be uninitialized in some test scenarios.
3.  **Test Environment Isolation**: Fixes widespread test failures in both `core` and `cli` packages by stubbing `GEMINI_CLI_HOME` in global test setups. This prevents CI-specific environment variables from interfering with home directory resolution in tests.
4.  **Lint Fix**: Resolves a lint error in `models_auth.test.ts` caused by using the spread operator on a class-cast object.

Fixes: #26857

cc @NTaylorMullen

packages/cli/test-setup.ts

@@ -56,6 +56,9 @@ let errorSpy: vi.SpyInstance;
 let debugSpy: vi.SpyInstance;
 
 beforeEach(() => {
+  // Ensure a clean home directory state for all tests
+  vi.stubEnv('GEMINI_CLI_HOME', '');
+
   // Reset themeManager state to ensure test isolation
   themeManager.resetForTesting();
 

packages/core/src/config/config.ts

@@ -2802,7 +2802,7 @@ export class Config implements McpContext, AgentLoopContext {
   }
 
   getAuthType(): string | undefined {
-    return this.contentGeneratorConfig.authType;
+    return this.contentGeneratorConfig?.authType;
   }
 
   getPendingIncludeDirectories(): string[] {

packages/core/src/config/models_auth.test.ts

@@ -10,17 +10,17 @@ import {
   PREVIEW_GEMINI_MODEL_AUTO,
   PREVIEW_GEMINI_MODEL,
   DEFAULT_GEMINI_MODEL,
+  type ModelCapabilityContext,
 } from './models.js';
 import { ModelConfigService } from '../services/modelConfigService.js';
 import { DEFAULT_MODEL_CONFIGS } from './defaultModelConfigs.js';
-import type { Config } from './config.js';
 
 const modelConfigService = new ModelConfigService(DEFAULT_MODEL_CONFIGS);
 
 const dynamicConfig = {
   getExperimentalDynamicModelConfiguration: () => true,
   modelConfigService,
-} as unknown as Config;
+} as unknown as ModelCapabilityContext;
 
 describe('resolveModel with authType', () => {
   it('should resolve auto-gemini-3 to gemini-3-pro-preview for non-personal auth', () => {
@@ -55,7 +55,7 @@ describe('resolveModel with authType', () => {
     const configWithAuth = {
       ...dynamicConfig,
       getAuthType: () => 'oauth-personal',
-    } as unknown as Config;
+    } as unknown as ModelCapabilityContext;
 
     const model = resolveModel(
       PREVIEW_GEMINI_MODEL_AUTO,

packages/core/test-setup.ts

@@ -10,7 +10,7 @@ if (process.env.NO_COLOR !== undefined) {
 }
 
 import { setSimulate429 } from './src/utils/testUtils.js';
-import { vi, afterEach } from 'vitest';
+import { vi, afterEach, beforeEach } from 'vitest';
 import { coreEvents } from './src/utils/events.js';
 
 // Increase max listeners to avoid warnings in large test suites
@@ -19,6 +19,11 @@ coreEvents.setMaxListeners(100);
 // Disable 429 simulation globally for all tests
 setSimulate429(false);
 
+beforeEach(() => {
+  // Ensure a clean home directory state for all tests
+  vi.stubEnv('GEMINI_CLI_HOME', '');
+});
+
 afterEach(() => {
   vi.unstubAllEnvs();
 });