Return 400 for invalid UTF-8 multipart fields

[lovasoa]

Summary

• return 400 Bad Request when a multipart text field contains invalid UTF-8
• add a regression test covering the curl repro for malformed multipart payloads
• document the fix in CHANGELOG.md

Verification

• cargo fmt --all
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test test_invalid_utf8_multipart_text_field_returns_bad_request -- --nocapture
• cargo test test_file_upload_direct -- --nocapture

Repro

curl --http1.1 -X POST http://127.0.0.1:6281/tests/requests/variables.sql -H 'Content-Type: multipart/form-data; boundary=xxx' --data-binary @/tmp/sqlpage-invalid-multipart.bin