chore: Fix the long running test cases

The test test_token_exchange_not_supported was slow because of an incorrect monkeypatch target. The test was patching google.adk.auth.auth_handler.AUTHLIB_AVAILABLE, but the actual OAuth2 exchange logic uses a different AUTHLIB_AVAILABLE variable in google.adk.auth.exchanger.oauth2_credential_exchanger.
What was happening:
Test set auth_handler.AUTHLIB_AVAILABLE = False
AuthHandler.exchange_auth_token() called OAuth2CredentialExchanger.exchange()
But oauth2_credential_exchanger.AUTHLIB_AVAILABLE was still True
The exchanger attempted real OAuth2 token exchange with client.fetch_token()
This made actual network calls to OAuth2 endpoints, causing timeouts and delays

PiperOrigin-RevId: 788576949

Author: seanzhougoogle

tests/unittests/auth/test_auth_handler.py

@@ -456,7 +456,10 @@ async def test_token_exchange_not_supported(
       self, auth_config_with_auth_code, monkeypatch
   ):
     """Test when token exchange is not supported."""
-    monkeypatch.setattr("google.adk.auth.auth_handler.AUTHLIB_AVAILABLE", False)
+    monkeypatch.setattr(
+        "google.adk.auth.exchanger.oauth2_credential_exchanger.AUTHLIB_AVAILABLE",
+        False,
+    )
 
     handler = AuthHandler(auth_config_with_auth_code)
     result = await handler.exchange_auth_token()