Use the offset buffer for Map in spark_size

[0lai0]

What is the problem the feature request solves?

Follow-up from #5233
spark_size for List / LargeList / FixedSizeList now reuses Arrow's length kernel.
The Map branch in native/spark-expr/src/array_funcs/size.rs is still a per-row loop that appends MapArray::value_length(i) into an Int32Builder.
length does not accept MapArray, so Map cannot reuse the kernel. It was kept out of #5233 on purpose: different technique, and its own correctness and benchmark surface.

Describe the potential solution

MapArray::offsets() is the entry-range offset buffer; for each row, offsets[i+1] - offsets[i] is the entry count (same value value_length(i) returns one row at a time). Vectorize that and apply Spark's null → -1 rewrite with the same set_indices patch the list path uses

Additional context

Before merging:

• Add a Map shape (with and without nulls) to benches/array_size.rs — there is no Map coverage today. The win is expected to be smaller than for List, since value_length already reads the offsets; only per-row dispatch and builder overhead go away. If the benchmark comes back flat, closing this as not worth the churn is fine.
• Add a sliced-MapArray test, mirroring the sliced-List test added in perf: compute spark_size list lengths with Arrow length kernel #5233. MapArray::offsets() returns the sliced offsets so windows(2) stays correct, but pin the invariant.

[0lai0]

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