GH-51271: [Ruby] Fix Date values in ArrowFormat::Date32Array

[emecii]

Rationale for this change

ArrowFormat::Date32Array currently stores Date#day, the day of the month, for Ruby Date values. Arrow Date32 values are signed day offsets from the Unix epoch, so this can silently produce incorrect data. The same conversion is used by ArrowFormat::Array.build when it infers Date32.

What changes are included in this PR?

• Convert Ruby Date values from their Julian day to the Unix epoch day offset.
• Cover dates before, at, and after the Unix epoch, including nulls.
• Make the ArrayBuilder regression assert the encoded payload instead of comparing two arrays built through the same conversion path.

Are these changes tested?

Yes. The full red-arrow-format suite passes locally: 691 tests and 695 assertions. The official rake runner used the current pure-Ruby sources with the locally installed Arrow 25.0.1 native extension; the local system does not have the Arrow C++ 26 dependency needed to build the native Ruby extension from main.

Are there any user-facing changes?

Yes. Ruby Date values now produce specification-compliant Date32 day offsets.

This PR contains a "Critical Fix". It fixes incorrect Date32 data produced from Ruby Date values.

AI assistance

AI assistance was used to investigate the defect, draft the focused implementation and tests, and run the verification described above. No human review is claimed.

• GitHub Issue: [Ruby] Fix Date values in ArrowFormat::Date32Array #51271

[kou]

+1