test(internal): expand coverage for compression_pipeline (#1041)

Add tests targeting branch coverage gaps in compression_pipeline:

- LZ4 decompress error surfaces: too-short header, oversized header,
  garbage payload, truncated valid payload, max-allowed header limit
- Zlib gzip/deflate growth-loop coverage with highly compressible
  large payloads that force the inflate buffer to grow
- Zlib gzip/deflate decompress error surfaces: garbage stream,
  truncated stream
- Helper function error-fallback paths for decompress with garbage
  and empty inputs
- Span overload coverage at threshold boundaries for all algorithms
- 'Compressed not smaller than original' branch with high-entropy
  small inputs across LZ4/gzip/deflate
- Repeated compress/decompress stability and cross-algorithm
  threshold switching

Closes #1029

Author: kcenon