Classify PostgreSQL response errors for logging

[JasonAtClockwork]

Follow-up from #5640.

crates/pg/src/pg_server.rs, response() combines expected client SQL errors with internal database failures. Preserve enough error classification for expected client failures to log at warn or lower while internal failures remain at error.

[UllasSHR]

Hey! I’ve been looking into this and testing it locally. I used a small Rust test project to run the PG response() function and saw that it logs a 500 response as a warning.

I made a small change to log it as an error, and the test passes while keeping the tested 400 behaviour unchanged. I haven’t run the full crate tests yet.

I noticed #5869 is working on the earlier SQL error classification. Would a separate PR for this PG logging change be useful, or should these changes be handled together?