[codex] fix: validate advertised API version ranges

[Herrtian]

Summary

• Validate ApiVersionsResponse api_key, min_version, and max_version values before narrowing them to i16.
• Return Error::UnsupportedVersion with the offending field name and value instead of panicking on malformed server metadata.
• Propagate validation failures from the API versions handshake.

Validation

• cargo fmt --check
• git diff --check
• cargo check -p fluss-rs
• cargo test -p fluss-rs server_api_versions --lib

Closes #535