Return a generic 503 message when the metastore fails during authentication

[shyundev]

Follow-up to #5247, as agreed there: a client that is not yet authenticated should not learn which metastore lookup failed. The three 503s raised during authentication returned Unable to fetch principal entity, Unable to fetch principal grants and Unable to fetch securable entity; all three now return a fixed Service unavailable.

The suggestion there was to log the detail against a UUID handed back to the client. Nothing new has to be minted: the response already carries the request id as X-Request-ID, and the default log format prints the same id as requestId. That is the route #4406 took for the 403 path.

The three DefaultAuthenticatorTest metastore-failure tests now assert the response message.

Checklist

• 🛡️ Don't disclose security issues! (contact security@apache.org)
• 🔗 Clearly explained why the changes are needed, or linked related issues: Follow-up to Return 503 instead of 500 when the metastore fails during authentication #5247
• 🧪 Added/updated tests with good coverage, or manually tested (and explained how)
• 💡 Added comments for complex logic
• 🧾 Updated CHANGELOG.md (if needed)
• 📚 Updated documentation in site/content/in-dev/unreleased (if needed)