[core] Improve BUCKET_APPEND_ORDERED test coverage

[dwangatt]

Purpose

Extracted from #9370, which is being split into smaller, independently reviewable PRs.

This PR contains only test coverage improvements for BUCKET_APPEND_ORDERED. It is independent of the per-partition bucket-count feature.

Changes

• Retain the existing restore-failure coverage when manifest files are unavailable.
• Add focused sequence-number assertions for ordered and unordered bucketed append writes:
  • ordered mode restores previous files and continues sequence numbers;
  • unordered write-only mode skips restore and starts sequence numbers from zero.

Testing

• AppendOnlySimpleTableTest#testBucketedAppendOrderedSequenceNumbers
• AppendOnlySimpleTableTest#testBucketedAppendUnorderedSequenceNumbers

Related

• Extracted from [WIP] Split partition-level bucket-count support into focused PRs #9370

[JingsongLi]

Requirement fit: SUPPORTED. The two-commit test checks an observable file contract: ordered bucketed append continues sequence numbers, while unordered write-only mode starts a new sequence. This covers behavior that the existing missing-manifest test does not assert. Implementation: CLEAN in this test-only diff. I did not rerun the core test locally.