feat: add token_endpoint_auth_method support to OAuth2 credentials

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

tests/unittests/auth/test_oauth2_credential_util.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import time
+from typing import Optional
 from unittest.mock import Mock
 
 from authlib.oauth2.rfc6749 import OAuth2Token
@@ -25,6 +26,39 @@ from google.adk.auth.auth_credential import OAuth2Auth
 from google.adk.auth.auth_schemes import OpenIdConnectWithConfig
 from google.adk.auth.oauth2_credential_util import create_oauth2_session
 from google.adk.auth.oauth2_credential_util import update_credential_with_tokens
+import pytest
+
+
+@pytest.fixture
+def openid_connect_scheme() -> OpenIdConnectWithConfig:
+  """Fixture providing a standard OpenIdConnectWithConfig scheme."""
+  return OpenIdConnectWithConfig(
+      type_="openIdConnect",
+      openId_connect_url="https://example.com/.well-known/openid_configuration",
+      authorization_endpoint="https://example.com/auth",
+      token_endpoint="https://example.com/token",
+      scopes=["openid", "profile"],
+  )
+
+
+def create_oauth2_auth_credential(
+    auth_type=AuthCredentialTypes.OPEN_ID_CONNECT,
+    token_endpoint_auth_method: Optional[str] = None,
+):
+  """Helper function to create OAuth2Auth credential with optional token_endpoint_auth_method."""
+  oauth2_auth = OAuth2Auth(
+      client_id="test_client_id",
+      client_secret="test_client_secret",
+      redirect_uri="https://example.com/callback",
+      state="test_state",
+  )
+  if token_endpoint_auth_method is not None:
+    oauth2_auth.token_endpoint_auth_method = token_endpoint_auth_method
+
+  return AuthCredential(
+      auth_type=auth_type,
+      oauth2=oauth2_auth,
+  )
 
 
 class TestOAuth2CredentialUtil:
@@ -41,14 +75,9 @@ class TestOAuth2CredentialUtil:
         token_endpoint="https://example.com/token",
         scopes=["openid", "profile"],
     )
-    credential = AuthCredential(
-        auth_type=AuthCredentialTypes.OPEN_ID_CONNECT,
-        oauth2=OAuth2Auth(
-            client_id="test_client_id",
-            client_secret="test_client_secret",
-            redirect_uri="https://example.com/callback",
-            state="test_state",
-        ),
+    credential = create_oauth2_auth_credential(
+        auth_type=AuthCredentialTypes.OAUTH2,
+        token_endpoint_auth_method="client_secret_jwt",
     )
 
     client, token_endpoint = create_oauth2_session(scheme, credential)
@@ -122,6 +151,62 @@ class TestOAuth2CredentialUtil:
     assert client is None
     assert token_endpoint is None
 
+  @pytest.mark.parametrize(
+      "token_endpoint_auth_method, expected_auth_method",
+      [
+          ("client_secret_post", "client_secret_post"),
+          (None, "client_secret_basic"),
+      ],
+  )
+  def test_create_oauth2_session_with_token_endpoint_auth_method(
+      self,
+      openid_connect_scheme,
+      token_endpoint_auth_method,
+      expected_auth_method,
+  ):
+    """Test create_oauth2_session with various token_endpoint_auth_method settings."""
+    credential = create_oauth2_auth_credential(
+        token_endpoint_auth_method=token_endpoint_auth_method
+    )
+
+    client, token_endpoint = create_oauth2_session(
+        openid_connect_scheme, credential
+    )
+
+    assert client is not None
+    assert token_endpoint == "https://example.com/token"
+    assert client.client_id == "test_client_id"
+    assert client.client_secret == "test_client_secret"
+    assert client.token_endpoint_auth_method == expected_auth_method
+
+  def test_create_oauth2_session_oauth2_scheme_with_token_endpoint_auth_method(
+      self,
+  ):
+    """Test create_oauth2_session with OAuth2 scheme and token_endpoint_auth_method."""
+    flows = OAuthFlows(
+        authorizationCode=OAuthFlowAuthorizationCode(
+            authorizationUrl="https://example.com/auth",
+            tokenUrl="https://example.com/token",
+            scopes={"read": "Read access", "write": "Write access"},
+        )
+    )
+    scheme = OAuth2(type_="oauth2", flows=flows)
+    credential = AuthCredential(
+        auth_type=AuthCredentialTypes.OAUTH2,
+        oauth2=OAuth2Auth(
+            client_id="test_client_id",
+            client_secret="test_client_secret",
+            redirect_uri="https://example.com/callback",
+            token_endpoint_auth_method="client_secret_jwt",
+        ),
+    )
+
+    client, token_endpoint = create_oauth2_session(scheme, credential)
+
+    assert client is not None
+    assert token_endpoint == "https://example.com/token"
+    assert client.token_endpoint_auth_method == "client_secret_jwt"
+
   def test_update_credential_with_tokens(self):
     """Test update_credential_with_tokens function."""
     credential = AuthCredential(