fix: decoding of binary data with null bytes

[kateweaver]

Problem

Decoding binary data (e.g. Data Matrix codes encoded with Base256 that include null bytes) fails in pylibdmtx because the decode wrapper treats it as string-like and truncates the output at the first \x00. This prevents accurate round-tripping of binary payloads.

This behavior is not present in the underlying libdmtx C library, which handles null bytes correctly.

Evidence

• Stack Overflow: https://stackoverflow.com/questions/67692547/error-decoding-data-matrix-that-is-base256-encoded-with-pylibdmtx
• Debian bug #1024731: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1024731

Fix

• fixes bug: decode() truncates Base256 binary data at null byte #109
• fixes an issue where pylibdmtx.decode() trucates data at first null byte causing binary payloads (base256 encoded) to decode incorrectly
• The underlying C library (libdmtx) supports null bytes correclty.
• The Python wrapper used C strings as-si, which results in null-termination.
• Replaces string_at(msg.contents.output)
• Now uses string_at(msg.contents.output, msg.contents.outputIdx)
• Tests added to verify null handling in both binary and ascii data.

Tests

• Added a test with binary and ascii payload including null byte.
• Confirmed failure before fix.
• Confirmed all tests pass after fix.

Additional Notes

• I noticed there are 2 other outstanding PRs proposing to fix the same issue:
  • Added optional argument to decode method to allow binary data decoded #45
  • decode: ensure full output is returned as a string. #54