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
gemini-cli[bot]