Return the OAuth task id from the MCP test route

The client follows an MCP sign-in by its task's events, but the test route
kept only success, requires_oauth and auth_url, so the wizard and the custom
MCP modal reported a failed sign-in before the popup could load.

tests/api/user/test_tools_mcp_pg.py

@@ -232,6 +232,8 @@ class TestTestMCPServerConfig:
             "success": False,
             "requires_oauth": True,
             "auth_url": "https://auth/ex",
+            "task_id": "task-123",
+            "message": "OAuth required",
         }
 
         with patch(
@@ -252,6 +254,8 @@ class TestTestMCPServerConfig:
             response = TestMCPServerConfig().post()
         assert response.status_code == 200
         assert response.json["requires_oauth"] is True
+        # The client follows the sign-in by this task's events.
+        assert response.json["task_id"] == "task-123"
 
     def test_unexpected_exception_returns_500(self, app):
         from docsgpt.api.user.tools.mcp import TestMCPServerConfig