fix(ipc): Decode a DictionaryEncoding without indexType as int32 indices

[CaptainAni187]

Closes #956

Schema.fbs says that when DictionaryEncoding.indexType is null, the indices must be signed int32. The decoder passed the missing table straight to ArrowIpcDecoderSetTypeInt(), which crashed (a segfault in release builds, flatcc's "null pointer table access" assertion in debug builds). It now uses int32 when the field isn't present.

The new NanoarrowIpcDecodeDictionarySchemaWithoutIndexType test decodes a schema message with one dictionary<values=string, indices=int32> field that pyarrow 25.0.1 wrote, with indexType then removed from it. It crashes without the fix. I also read the WITHOUT_INDEX_TYPE stream from the issue with ArrowIpcArrayStreamReader: it now gives struct<text: dictionary(int32)<string>> with the values a b a, the same as the stream that has indexType.

ctest passes (384 tests, compression tests skipped), and pre-commit passes on the changed files.

[paleolimbot]

On green CI this is good to merge...thank you!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.20%. Comparing base (ecba475) to head (af4ef30).

Files with missing lines	Patch %	Lines
src/nanoarrow/ipc/decoder.c	33.33%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #957      +/-   ##
==========================================
- Coverage   78.20%   78.20%   -0.01%     
==========================================
  Files         106      106              
  Lines       16849    16851       +2     
  Branches     1986     1988       +2     
==========================================
+ Hits        13177    13178       +1     
  Misses       2438     2438              
- Partials     1234     1235       +1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.