fix(tests): Remove session-scoped fixture overrides from MCP conftest

The MCP conftest.py was overriding session-scoped `server` and
`graph_cleanup` fixtures with no-op versions. Having two session-scoped
fixtures with the same name at different directory levels caused
pytest-asyncio event loop conflicts, making all oauth_test.py tests
fail with "Event loop is closed".

Since these fixtures are session-scoped and shared across the entire
test run, the override was unnecessary — the SpinTestServer is already
created for other tests.

Also adds defensive `access_token` key validation in MCP OAuth token
exchange and refresh to prevent KeyError on malformed responses.

autogpt_platform/backend/backend/blocks/mcp/conftest.py

@@ -1,13 +1,11 @@
 """
 Conftest for MCP block tests.
 
-Override the session-scoped server and graph_cleanup fixtures from
-backend/conftest.py so that MCP integration tests don't spin up the
-full SpinTestServer infrastructure.
+Registers the e2e marker and --run-e2e CLI option so MCP end-to-end tests
+(which hit real external servers) can be gated behind a flag.
 """
 
 import pytest
-import pytest_asyncio
 
 
 def pytest_configure(config: pytest.Config) -> None:
@@ -29,15 +27,3 @@ def pytest_addoption(parser: pytest.Parser) -> None:
     parser.addoption(
         "--run-e2e", action="store_true", default=False, help="run e2e tests"
     )
-
-
-@pytest_asyncio.fixture(scope="session", loop_scope="session")
-async def server():
-    """No-op override — MCP tests don't need the full platform server."""
-    yield None
-
-
-@pytest_asyncio.fixture(scope="session", loop_scope="session", autouse=True)
-async def graph_cleanup(server):
-    """No-op override — MCP tests don't create graphs."""
-    yield

autogpt_platform/backend/backend/blocks/mcp/oauth.py

@@ -109,6 +109,11 @@ class MCPOAuthHandler(BaseOAuthHandler):
                 f"Token exchange failed: {tokens.get('error_description', tokens['error'])}"
             )
 
+        if "access_token" not in tokens:
+            raise RuntimeError(
+                "OAuth token response missing 'access_token' field"
+            )
+
         now = int(time.time())
         expires_in = tokens.get("expires_in")
 
@@ -158,6 +163,11 @@ class MCPOAuthHandler(BaseOAuthHandler):
                 f"Token refresh failed: {tokens.get('error_description', tokens['error'])}"
             )
 
+        if "access_token" not in tokens:
+            raise RuntimeError(
+                "OAuth refresh response missing 'access_token' field"
+            )
+
         now = int(time.time())
         expires_in = tokens.get("expires_in")