fix(ipc): handle missing flatbuffer tables and dictionary flags

[paleolimbot]

Reject IPC messages with missing Field.type, Message.header, or DictionaryBatch.data tables. Validate nested dictionary record-batch node and buffer counts before accessing their vectors, returning EINVAL for missing or mismatched vectors. Preserve outer-field nullability while keeping dictionary values nullable.

Add regressions for malformed tables, missing/empty/incorrect vector counts through both dictionary decoding APIs, recovery after rejected batches, default int32 dictionary indices, and non-nullable dictionary round trips.

Closes #958
Closes #959

Tests: 102 IPC-related CTests passed with ZSTD and LZ4 enabled; pre-commit checks passed. The new vector-count regression reproduces the assertion failure before the fix.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.96970% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.24%. Comparing base (bb5d8d7) to head (37ccebf).

Files with missing lines	Patch %	Lines
src/nanoarrow/ipc/decoder.c	96.96%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #960      +/-   ##
==========================================
+ Coverage   78.20%   78.24%   +0.04%     
==========================================
  Files         106      106              
  Lines       16851    16867      +16     
  Branches     1988     1993       +5     
==========================================
+ Hits        13178    13198      +20     
+ Misses       2438     2437       -1     
+ Partials     1235     1232       -3     

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