[rust] Return error for out-of-range server API versions

[slfan1989]

Purpose

Linked issue: close #4241

ServerApiVersions::new() converted the api_key, min_version, and max_version fields received from the server from i32 to i16 using unwrap().

An out-of-range value in the server's ApiVersionsResponse could therefore panic the Rust client during connection establishment.

This change makes invalid server API version values return a descriptive error instead of causing a panic.

Brief change log

• Change ServerApiVersions::new() to return Result<ServerApiVersions, Error>.
• Replace the integer conversion unwrap() calls with checked conversions.
• Include the invalid field name and value in the returned error.
• Preserve the conversion error as the error source.
• Propagate validation errors through the API version handshake.
• Add unit tests for out-of-range:
  • api_key
  • min_version
  • max_version

Tests

Manually ran the unit tests.

cargo test -p fluss-rs --all-targets

API and Format

No.

Documentation

No.