GH-31387: [C++] Check nullability when validating fields on batches or struct arrays

[singh1203]

Rationale for this change

Ensures schema validation catches null values in non-nullable fields, preventing silent errors when writing to formats like Parquet.

What changes are included in this PR?

Fixes: #31387

• Nullability checks were added in ValidateFull() for arrays, struct arrays, union arrays, and record batches.
• Introduced new validation logic in validate.cc to recursively check for nulls in non-nullable fields.
• Added unit tests in:
  • array_test.cc
  • array_struct_test.cc
  • array_union_test.cc
  • record_batch_test.cc

These tests ensure that ValidateFull() fails when nulls are present in non-nullable fields.

Are these changes tested?

Yes, new unit tests have been added

Are there any user-facing changes?

Yes, Users who try to validate or write Arrow data with nulls in non-nullable fields will now receive an explicit validation error.

• GitHub Issue: #31387

[github-actions[bot]]

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See also:

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• Contribution Guidelines - Contributing Overview

[github-actions[bot]]

:warning: GitHub issue #31387 has been automatically assigned in GitHub to PR creator.

[singh1203]

cc: @pitrou @lidavidm