fix: LZ4 block compressor/decompressor safety issues

[lxy-9602]

Purpose

No Linked issue.

• Decompressor: Add src_length < HEADER_LENGTH guard before ReadIntLE to prevent out-of-bounds read on truncated/corrupted blocks.
• Compressor: Change compressed_size < 0 to <= 0 since LZ4_compress_default returns 0 (not negative) on failure, which previously produced silent corrupt blocks.

Tests

• TestDecompressTruncatedHeader: Verifies truncated input (3 bytes / 0 bytes) returns Invalid instead of reading out-of-bounds.
• TestCompressInsufficientOutputBuffer: Verifies insufficient output buffer returns error instead of writing a corrupt block.

API and Format

Documentation

Generative AI tooling

[zjw1111]

Re-reviewed the latest head and the previous LZ4 output-buffer issue is fixed. No remaining actionable issues from my side.