Merge https://github.com/google/adk-python/pull/2870
## Summary
Add `token_endpoint_auth_method` field to OAuth2Auth class to allow configuring OAuth2 token endpoint authentication methods. This enables users to specify how the client should authenticate with the authorization server's token
endpoint.
• Add `token_endpoint_auth_method` field to `OAuth2Auth` with default value `"client_secret_basic"`
• Update `create_oauth2_session()` to pass the authentication method to `OAuth2Session`
• Maintain backward compatibility with existing OAuth2 configurations
## Unit Tests
Added unit test coverage with 3 new test methods:
1. `test_create_oauth2_session_with_token_endpoint_auth_method()` - Tests explicit auth method setting (`client_secret_post`)
2. `test_create_oauth2_session_with_default_token_endpoint_auth_method()` - Tests default behavior (`client_secret_basic`)
3. `test_create_oauth2_session_oauth2_scheme_with_token_endpoint_auth_method()` - Tests with OAuth2 scheme using `client_secret_jwt`
**Test Results:**
✅ 16/16 OAuth2 credential utility tests passed
✅ 240/240 auth module tests passed (no regressions)
✅ Tests cover both GOOGLE_AI and VERTEX variants
✅ Pylint score: 9.41/10
## Changes Made
**src/google/adk/auth/auth_credential.py**
- Added `token_endpoint_auth_method: Optional[str] = "client_secret_basic"` to `OAuth2Auth` class
**src/google/adk/auth/oauth2_credential_util.py**
- Updated `create_oauth2_session()` to pass `token_endpoint_auth_method` parameter to `OAuth2Session`
**tests/unittests/auth/test_oauth2_credential_util.py**
- Added 3 comprehensive test methods covering different authentication scenarios
## Backward Compatibility
✅ **Non-breaking change** - All existing OAuth2 configurations continue to work unchanged with the default `client_secret_basic` authentication method.
## Supported Authentication Methods
- `client_secret_basic` (default) - Client credentials in Authorization header
- `client_secret_post` - Client credentials in request body
- `client_secret_jwt` - JWT with client secret
- `private_key_jwt` - JWT with private key
Co-authored-by: Xiang (Sean) Zhou <seanzhougoogle@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/2870 from sully90:feat/oauth2-token-endpoint-auth-method 04fe8244598f96b4e3366f0fc79382628382e9c2
PiperOrigin-RevId: 843739984
