fix: restore `does not exist` to validation error classification

[anandgupta42]

Problem

PR #611 removed "does not exist" from the validation error classification keywords to prevent HTTP 404 messages from being misclassified. However, the pattern reordering in the same PR (moving http_error before validation) already prevents that issue.

The removal caused SQL errors like "column foo does not exist" to fall through to unknown instead of validation.

Found by

Multi-model code review (Claude + Gemini 3.1 Pro consensus).

Fix

Restore "does not exist" to validation keywords. HTTP 404 errors still classify correctly as http_error because that pattern has priority.