test: mark management-api integration test with login_flow

`tests/server/login_flow/test_management_api.py` had
`[pytest.mark.integration, pytest.mark.oauth]` while every other test in
`tests/server/login_flow/` uses `[pytest.mark.integration,
pytest.mark.login_flow]`. The single-user CI matrix filter is
`(integration and not keycloak and not login_flow and not
multi_user_basic)`, so the missing `login_flow` mark let this test
collect and run under single-user mode against `localhost:8004` (which
isn't up there, hitting the bug being reported), even though it's
specifically driving the login-flow MCP server.

Also `oauth` isn't a registered marker (see `[tool.pytest.ini_options]`
in pyproject.toml), so it was emitting an unregistered-marker warning.

Replacing the marker aligns this file with its siblings: single-user /
multi-user-basic / keycloak filters all deselect it now, and the
login-flow filter still picks it up.

Verified: `pytest --collect-only -m "<single-user filter>"` reports 2
deselected; `-m login_flow` collects both tests.

tests/server/login_flow/test_management_api.py

@@ -15,7 +15,7 @@ import pytest
 
 LOGIN_FLOW_API_BASE_URL = "http://localhost:8004"
 
-pytestmark = [pytest.mark.integration, pytest.mark.oauth]
+pytestmark = [pytest.mark.integration, pytest.mark.login_flow]
 
 
 async def test_get_installed_apps_returns_capability_keys(