test(s3): cover empty-code no-such-key fallback

[overtrue]

Summary

This PR adds one focused regression test for the S3 Select error-classification path that treats an empty AWS error code the same as missing metadata.

The current test coverage already exercises None metadata for NoSuchKey and the empty-string fallback for AccessDenied, while open PR #158 covers the remaining empty-string NoSuchBucket and NotImplemented branches. One symmetric branch was still untested: if the SDK surfaces Some("") and the backend text contains NoSuchKey, the CLI should continue to classify the response as object-not-found instead of falling back to a generic error.

Root cause

The implementation already normalizes empty error codes with code.filter(|s| !s.is_empty()), so the runtime behavior was correct. The gap was only in regression coverage for that final empty-code NoSuchKey path.

Changes

• Add classify_empty_code_maps_no_such_key_substring in crates/s3/src/select.rs
• Keep runtime logic unchanged

Validation

• cargo test -p rc-s3 select::tests --lib
• make pre-commit