GH-46659: [C++] Fix export of extension arrays with binary view/string view storage

[paleolimbot]

Rationale for this change

Extension arrays exported with binary view/string view storage did not export the variadic sizes buffer which resulted in crashes when reimporting.

What changes are included in this PR?

The expression that controlled whether the variadic sizes buffer was written was updated.

Are these changes tested?

Yes, a test was added

Are there any user-facing changes?

No

• GitHub Issue: [C++] Exporting an ExtensionType with BinaryView or StringView storage to C does not export extra buffer sizes array #46659

[github-actions]

⚠️ GitHub issue #46659 has been automatically assigned in GitHub to PR creator.

[paleolimbot]

@bkietz Any chance you have a minute to ensure I understood this correctly?

[bkietz]

This looks right to me, just a few nits

[paleolimbot]

Thank you for the review!

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit 610c201.

There were 70 benchmark results with an error:

• Commit Run on arm64-t4g-2xlarge-linux at 2025-06-04 23:49:38Z
  • tpch (R) with engine=arrow, format=parquet, language=R, memory_map=False, query_id=TPCH-07, scale_factor=1
  • tpch (R) with engine=arrow, format=native, language=R, memory_map=False, query_id=TPCH-09, scale_factor=1
• and 68 more (see the report linked below)

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 4 possible false positives for unstable benchmarks that are known to sometimes produce them.