GET /auth/internal/users error when users_id omitted

[Ameb8]

Summary

GET /auth/internal/users returns 500 when user_ids is omitted, but the route declares user_ids as a required FastAPI
query parameter and should return 422.

Repro

curl -H "X-Api-Key: test-users-internal-api-key" \
  http://localhost:8101/auth/internal/users

## Expected

HTTP 422 from FastAPI required query validation.

## Actual

HTTP 500 with details similar to:

PydanticUndefinedType object is not iterable
vars() argument must have __dict__ attribute

## Notes

The route uses user_ids: list[int] = Query(...). FastAPI appears to detect the missing parameter, but the pinned FastAPI/
Pydantic stack fails while JSON-encoding the resulting RequestValidationError because it contains PydanticUndefined.

Integration coverage was added as a strict xfail so unrelated failures still fail normally, and the test should XPASS once
this bug is fixed.