cython_lz4: support configured frames above 256 MiB #1000

cassandra/cython_lz4.pyx rejects declared uncompressed sizes above 256 MiB. That value is a server default, not the native-protocol limit, so installing the preferred Cython codec breaks clusters configured for larger valid frames.

Validate against the signed 32-bit protocol/LZ4 API limit and add boundary coverage.

Found while reviewing #950; unrelated to that PR.