fix: Remove fallback to cached exchanged credential in _load_existing_credential

The _load_existing_credential method in CredentialManager will now only attempt to load credentials from the credential service and will no longer check the AuthConfig's exchanged_auth_credential cache.

Close #3772

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 852944533

Author: GWeale

src/google/adk/auth/credential_manager.py

@@ -176,17 +176,13 @@ async def get_auth_credential(
   async def _load_existing_credential(
       self, callback_context: CallbackContext
   ) -> Optional[AuthCredential]:
-    """Load existing credential from credential service or cached exchanged credential."""
+    """Load existing credential from credential service."""
 
     # Try loading from credential service first
     credential = await self._load_from_credential_service(callback_context)
     if credential:
       return credential
 
-    # Check if we have a cached exchanged credential
-    if self._auth_config.exchanged_auth_credential:
-      return self._auth_config.exchanged_auth_credential
-
     return None
 
   async def _load_from_credential_service(

tests/unittests/auth/test_credential_manager.py

@@ -133,7 +133,7 @@ async def test_load_auth_credentials_no_credential(self):
 
   @pytest.mark.asyncio
   async def test_load_existing_credential_already_exchanged(self):
-    """Test _load_existing_credential when credential is already exchanged."""
+    """Test _load_existing_credential ignores shared config cache."""
     auth_config = Mock(spec=AuthConfig)
     mock_credential = Mock(spec=AuthCredential)
     auth_config.exchanged_auth_credential = mock_credential
@@ -145,7 +145,7 @@ async def test_load_existing_credential_already_exchanged(self):
 
     result = await manager._load_existing_credential(callback_context)
 
-    assert result == mock_credential
+    assert result is None
 
   @pytest.mark.asyncio
   async def test_load_existing_credential_with_credential_service(self):