fix(auth): omit scope from OAuth2 token requests

- Stop setting scope on the shared create_oauth2_session helper
- Drops scope from both token exchange and refresh requests
- Neither needs scope (RFC 6749 4.1.3, 6); some providers reject it
- Auth URL construction in auth_handler.py keeps scope, unchanged
- Add body-level tests asserting refresh and exchange omit scope

Merge #5874

Change-Id: I78e7e1e69076afcc13460b7965ca05cae734814e

Author: doughayden

src/google/adk/auth/oauth2_credential_util.py

@@ -49,21 +49,18 @@ def create_oauth2_session(
       logger.warning("OpenIdConnect scheme missing token_endpoint")
       return None, None
     token_endpoint = auth_scheme.token_endpoint
-    scopes = auth_scheme.scopes or []
   elif isinstance(auth_scheme, OAuth2):
     # Support both authorization code and client credentials flows
     if (
         auth_scheme.flows.authorizationCode
         and auth_scheme.flows.authorizationCode.tokenUrl
     ):
       token_endpoint = auth_scheme.flows.authorizationCode.tokenUrl
-      scopes = list(auth_scheme.flows.authorizationCode.scopes.keys())
     elif (
         auth_scheme.flows.clientCredentials
         and auth_scheme.flows.clientCredentials.tokenUrl
     ):
       token_endpoint = auth_scheme.flows.clientCredentials.tokenUrl
-      scopes = list(auth_scheme.flows.clientCredentials.scopes.keys())
     else:
       logger.warning(
           "OAuth2 scheme missing required flow configuration. Expected either"
@@ -84,11 +81,12 @@ def create_oauth2_session(
   ):
     return None, None
 
+  # Scope is intentionally omitted: token exchange and refresh don't require
+  # it per RFC 6749, and some providers reject it on these requests.
   return (
       OAuth2Session(
           auth_credential.oauth2.client_id,
           auth_credential.oauth2.client_secret,
-          scope=" ".join(scopes),
           redirect_uri=auth_credential.oauth2.redirect_uri,
           state=auth_credential.oauth2.state,
           token_endpoint_auth_method=auth_credential.oauth2.token_endpoint_auth_method,

tests/unittests/auth/test_oauth2_credential_util.py

@@ -207,6 +207,82 @@ def test_create_oauth2_session_oauth2_scheme_with_token_endpoint_auth_method(
     assert token_endpoint == "https://example.com/token"
     assert client.token_endpoint_auth_method == "client_secret_jwt"
 
+  def _oauth2_scheme_with_scopes(self):
+    """Build an OAuth2 scheme that declares scopes."""
+    return OAuth2(
+        type_="oauth2",
+        flows=OAuthFlows(
+            authorizationCode=OAuthFlowAuthorizationCode(
+                authorizationUrl="https://example.com/auth",
+                tokenUrl="https://example.com/token",
+                scopes={"read": "Read access", "write": "Write access"},
+            )
+        ),
+    )
+
+  def _capturing_post(self, captured):
+    """Stub for OAuth2Session.post that records the token-request body."""
+
+    def _post(*args, **kwargs):
+      captured["data"] = kwargs.get("data")
+      response = Mock()
+      response.status_code = 200
+      response.json.return_value = {
+          "access_token": "new_access_token",
+          "token_type": "Bearer",
+          "expires_in": 3600,
+          "refresh_token": "new_refresh_token",
+      }
+      return response
+
+    return _post
+
+  def test_refresh_request_omits_scope(self):
+    """Refresh requests must not carry scope (some providers reject it)."""
+    credential = AuthCredential(
+        auth_type=AuthCredentialTypes.OAUTH2,
+        oauth2=OAuth2Auth(
+            client_id="test_client_id",
+            client_secret="test_client_secret",
+            redirect_uri="https://example.com/callback",
+        ),
+    )
+
+    client, token_endpoint = create_oauth2_session(
+        self._oauth2_scheme_with_scopes(), credential
+    )
+    assert client is not None
+
+    captured = {}
+    client.post = self._capturing_post(captured)
+    client.refresh_token(token_endpoint, refresh_token="old_refresh_token")
+
+    assert "scope" not in captured["data"]
+
+  def test_token_exchange_omits_scope(self):
+    """Authorization-code exchange must not carry scope (it is redundant)."""
+    credential = AuthCredential(
+        auth_type=AuthCredentialTypes.OAUTH2,
+        oauth2=OAuth2Auth(
+            client_id="test_client_id",
+            client_secret="test_client_secret",
+            redirect_uri="https://example.com/callback",
+        ),
+    )
+
+    client, token_endpoint = create_oauth2_session(
+        self._oauth2_scheme_with_scopes(), credential
+    )
+    assert client is not None
+
+    captured = {}
+    client.post = self._capturing_post(captured)
+    client.fetch_token(
+        token_endpoint, grant_type="authorization_code", code="test_code"
+    )
+
+    assert "scope" not in captured["data"]
+
   def test_update_credential_with_tokens(self):
     """Test update_credential_with_tokens function."""
     credential = AuthCredential(