[fix](paimon) Accept required Variant array fallback value

[Gabriel39]

Problem

Paimon can write an unannotated shredded Variant array whose element wrapper contains only a required BYTE_ARRAY value fallback leaf. Doris already recognizes Paimon's optional wrapper groups, but rejected this required fallback leaf during Parquet schema validation.

Changes

• Allow a required fallback leaf only for a Paimon unannotated, fallback-only array element.
• Keep annotated Variant layouts, object wrappers, and array elements with a typed carrier under the existing strict validation rules.
• Verify that the non-nullable fallback leaf is materialized into Variant array elements.

Validation

• ASAN BE unit tests: ParquetSchemaTest.*, VariantColumnReaderTest.*, and PaimonVariantReaderTest.* (82 tests passed).
• clang-format 16 validation on all affected C/C++ files.
• git diff --check.

Jira: DORIS-27886

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
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[Gabriel39]

run buildall

[Gabriel39]

/review

[github-actions]

Automated review opinion: approval. I found no blocking correctness issue in the reviewed diff.

Critical checkpoints:

• External compatibility and scope: the relaxation is reachable only through the explicit Paimon table-schema Variant override. It admits the unannotated fallback-only array-element layout with a required BYTE_ARRAY value, while annotated Parquet Variant groups, object-field wrappers, and array elements that also have typed_value retain strict validation.
• Schema, nested-level, and materialization correctness: table/file/leaf identities are unchanged. The optional array-element wrapper and required payload remain in their existing repetition/definition-level domains; valid payloads reconstruct the Variant element, while null or missing array wrappers still return corruption. No decoder cursor, lifecycle, cache, resource-ownership, or hot-path performance behavior changes.
• Negative compatibility cases: focused tests preserve rejection for a required object fallback, a required fallback beside a typed carrier, and the same required fallback in an annotated Variant layout.
• Tests: the new schema test reaches the formerly rejected validation branch, and the materialization test validates the reconstructed array values. Existing nested sparse-filter/cross-page coverage exercises the same level-domain behavior. A generated Paimon-file end-to-end case for this exact layout, including null/empty arrays and small page/batch limits, would be useful additional hardening but is not blocking based on the traced production path.
• Review focus and convergence: no additional user focus was supplied. The complete four-file PR was reviewed; both the normal full-review and separate risk-focused Round 1 scans returned NO_NEW_VALUABLE_FINDINGS, and no existing inline thread required duplicate suppression.

This review-only runner did not execute builds or tests; the conclusion is based on the authoritative diff, caller/implementation/test tracing, and the PR-reported validation.

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 100.00% (20/20) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	73.77% (30544/41407)
Line Coverage	57.82% (336807/582547)
Region Coverage	54.68% (280168/512405)
Branch Coverage	55.61% (125607/225868)