fix(ipc): Don't truncate metadata keys and values at NUL bytes

[rustyconover]

Closes #970.

ArrowIpcEncodeMetadata() wrote KeyValue keys and values with flatcc's _create_strn(). That function stops at the first NUL byte (flatcc_builder_create_string(B, s, strnlen(s, max_len))), so a metadata key or value containing a NUL was silently truncated in Schema, Field and Message custom_metadata. NUL (U+0000) is valid UTF-8, and both nanoarrow's metadata format and flatbuffers strings store a length with each string, so the encoder now uses the length-exact _create() to write exactly size_bytes bytes. The decoder already reads keys and values by length. No other IPC encoder code uses _create_strn().

The new NanoarrowIpcEncoderMetadataWithNulBytes test round-trips schema, field and message metadata whose keys and values contain NUL bytes, through the encoder/decoder and through ArrowIpcWriter/ArrowIpcArrayStreamReader, and checks that the bytes match exactly. It fails without the fix: ("field\0key", "\0field\0value") decoded as ("field", ""). ctest passes with and without Arrow C++, also under ASan/UBSan, and pre-commit passes on the changed files.

This PR's code and description were written by AI (Claude Code) and reviewed by a human maintainer.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.24%. Comparing base (e5f3d0a) to head (d83a538).

Files with missing lines	Patch %	Lines
src/nanoarrow/ipc/encoder.c	0.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #971   +/-   ##
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  Coverage   78.24%   78.24%           
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  Files         106      106           
  Lines       16901    16901           
  Branches     2012     2012           
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  Hits        13224    13224           
  Misses       2435     2435           
  Partials     1242     1242           

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