fix(ipc): Write the actual null count to each FieldNode

[rustyconover]

ArrowIpcEncoderCollectArray() copied ArrowArrayView.null_count into each FieldNode as it was. That is wrong in two cases:

• Unknown null count. In the C data interface, null_count may be -1 when it hasn't been computed. A zero-copy slice often looks like this. The IPC format has no such value: Message.fbs documents FieldNode.null_count as "the number of observed nulls", and Columnar.rst treats the null count as a property of the array that is "as large as the array length". The encoder wrote null_count: -1.
• Null type. Every slot of a null array is null, so its null count is its length. Arrow C++ enforces this (ValidateArray reports "Null array null_count unequal to its length", and NullArray forces null_count = length). A producer may still leave a null array's null_count at 0, and the encoder wrote that 0.

The encoder now writes the length for a null array. For other types it computes the null count with ArrowArrayViewComputeNullCount() when the count is negative. The change is in ArrowIpcEncoderCollectArray(), which also collects dictionary values, so it applies to DictionaryBatch messages as well. The top-level record batch check already handled -1.

I found this when a DuckDB extension started writing its IPC with nanoarrow and Apache.Arrow for .NET read the stream. Arrow C++ happens to accept both cases: it maps -1 to kUnknownNullCount and recomputes it, and NullArray overwrites the count. Apache.Arrow 23.0.0 (.NET) does not:

• null_count: -1 throws InvalidDataException: Null count of -1 must be >= 0 and <= 2147483647. Older versions read the column as having no nulls, so NULLs were silently lost.
• A null child with null_count: 0 throws ArgumentException: Length must equal null count from NullArray..ctor.

With this change, both batches read correctly in .NET.

Tests: NanoarrowIpcEncoderWritesActualNullCounts encodes struct<i: int32, n: null> with i = [1, null, 3], i.null_count = -1 and n.null_count = 0. It decodes the message and checks the field-node null counts, which must be 1 and 3. On main the test fails with -1 and 0. ctest passes (Linux aarch64, Debug, 393 tests), including under ASan/UBSan with clang. The changed files are formatted with clang-format 16.0.6, the pre-commit version.

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.30%. Comparing base (cc67c6b) to head (5c3cb53).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #974   +/-   ##
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  Coverage   78.29%   78.30%           
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  Files         106      106           
  Lines       16914    16919    +5     
  Branches     2014     2016    +2     
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+ Hits        13243    13248    +5     
  Misses       2429     2429           
  Partials     1242     1242           

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[rustyconover]

@paleolimbot sorry for doing a lot of cross language checks and finding these bugs.

I know .Net and C# isn't often run against nanoarrow, but I figured why not run the entire suite of arrow language conformance tests and these came out.