test(s3): cover select no-such-key fallback

[overtrue]

Summary

This PR closes a remaining regression gap in the recent S3 Select fallback classification coverage.

When an S3-compatible backend fails to return structured error metadata, crates/s3/src/select.rs falls back to matching known error substrings in the response text. The NoSuchKey branch in that fallback logic is part of the current runtime behavior, but it was still untested after the recent Select coverage additions.

This patch adds one focused unit test for the metadata-missing NoSuchKey path. Runtime behavior is unchanged.

Root Cause

Recent Select follow-up PRs covered serialization helpers, exit codes, AccessDenied, and NotImplemented fallback handling, but they did not exercise the NoSuchKey substring fallback that maps missing-object responses to NotFound.

Validation

• cargo test -p rc-s3 classify_missing_code_maps_no_such_key_substring --lib
• make pre-commit