feat: Support refresh access token automatically for rest_api_tool

1. let auth_handler.py to utilize the oauth2 credential fetcher to exchange token
2. restructure tool_auth_handler.py to support refresh token

PiperOrigin-RevId: 770901469

tests/unittests/auth/test_auth_handler.py

@@ -449,7 +449,7 @@ class TestExchangeAuthToken:
   ):
     """Test when token exchange is not supported."""
     monkeypatch.setattr(
-        "google.adk.auth.auth_handler.SUPPORT_TOKEN_EXCHANGE", False
+        "google.adk.auth.oauth2_credential_fetcher.AUTHLIB_AVIALABLE", False
     )
 
     handler = AuthHandler(auth_config_with_auth_code)
@@ -537,7 +537,10 @@ class TestExchangeAuthToken:
 
     assert result == oauth2_credentials_with_token
 
-  @patch("google.adk.auth.auth_handler.OAuth2Session", MockOAuth2Session)
+  @patch(
+      "google.adk.auth.oauth2_credential_fetcher.OAuth2Session",
+      MockOAuth2Session,
+  )
   def test_successful_token_exchange(self, auth_config_with_auth_code):
     """Test a successful token exchange."""
     handler = AuthHandler(auth_config_with_auth_code)