fix(oauth): add client id to token exchange

Merge https://github.com/google/adk-python/pull/2805

fixes #2806
add client id to token request, adhering to [RFC 6749](https://datatracker.ietf.org/doc/html/rfc6749#section-4.1.3)

Co-authored-by: Xuan Yang <xygoogle@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/2805 from davidkl97:fix/oauth-exchange-token 3366dce65ce4dd4fee649a81ab7e486689637442
PiperOrigin-RevId: 840369113

src/google/adk/auth/exchanger/oauth2_credential_exchanger.py

@@ -196,6 +196,7 @@ class OAuth2CredentialExchanger(BaseCredentialExchanger):
           ),
           code=auth_credential.oauth2.auth_code,
           grant_type=OAuthGrantType.AUTHORIZATION_CODE,
+          client_id=auth_credential.oauth2.client_id,
       )
       update_credential_with_tokens(auth_credential, tokens)
       logger.debug("Successfully exchanged authorization code for access token")

tests/unittests/auth/exchanger/test_oauth2_credential_exchanger.py

@@ -341,6 +341,7 @@ class TestOAuth2CredentialExchanger:
         authorization_response="https://example.com/callback?code=auth_code",  # Normalized URI
         code="auth_code",
         grant_type=OAuthGrantType.AUTHORIZATION_CODE,
+        client_id="test_client_id",
     )
 
   @pytest.mark.asyncio