Implement gunzip function for GZIP decompression

[staging-devin-ai-integration]

Implement gunzip function for GZIP decompression

Summary

Adds a new gunzip function that decompresses GZIP-compressed input streams into strings. The function takes std::istream &stream as input and returns std::optional<std::string>, following the same pattern and error handling approach as the existing gzip compression function.

Key changes:

• Added gunzip function declaration to header with comprehensive documentation and usage example
• Implemented stream-based decompression using zlib's inflate functions as counterpart to existing deflate usage
• Added 4 comprehensive test cases covering basic decompression, round-trip testing, invalid data handling, and empty stream handling
• Uses same 4096-byte buffer size and error handling patterns as existing gzip function

Review & Testing Checklist for Human

This is a medium-risk change due to low-level zlib usage and stream handling complexity. Please verify:

• Stream handling correctness: Test the function with various sizes of compressed data (small, large, multi-chunk) to ensure the stream reading loop handles partial reads and buffer boundaries correctly
• Error handling robustness: Try feeding malformed/corrupted GZIP data to verify all error paths return std::nullopt appropriately and don't crash or leak memory
• Round-trip compatibility: Verify that data compressed with the existing gzip function can be reliably decompressed with gunzip across different data types and sizes
• Resource management: Check for memory leaks by running tests under valgrind or similar tool, especially focusing on error paths where inflateEnd() must be called

Test Plan Recommendation

// Test with your own data beyond the unit tests
const std::string test_data = "Your test data here...";
const auto compressed = sourcemeta::core::gzip(test_data);
std::istringstream stream(compressed.value());
const auto result = sourcemeta::core::gunzip(stream);
assert(result.value() == test_data);

Notes

• Implementation mirrors the existing gzip function structure using zlib's inflate functions instead of deflate
• Added has_data flag to properly handle empty stream case (this was needed to fix failing tests)
• All error conditions return std::nullopt following the existing pattern
• Link to Devin run: https://staging.itsdev.in/sessions/72d4a78026d24b5e9dbd4009e1c3b035
• Requested by: Juan Cruz Viotti (@jviotti)

[staging-devin-ai-integration]

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