Fix Gmail multi-label filtering

[Bbbstin]

Summary

• fix messages.list returning 500 Internal Server Error for multi-label requests such as GET /gmail/v1/users/me/messages?labelIds=INBOX&labelIds=CATEGORY_UPDATES
• replace repeated unaliased MessageLabel joins, which produced ambiguous SQL, with per-label relationship any() checks that require every requested label
• add regression coverage confirming that only messages containing every requested label are returned

Validation

• cd packages/environments/mock-gmail && uv run --extra dev pytest tests -q (247 passed)
• git diff --check

Notes

• no API contract or database schema changes