[FLINK] Improve Flink sink append and bucket partitioner test coverage

[cshuo]

Summary

Improve unit-test coverage for the Flink sink write path, especially append buffer variants and dynamic bucket assignment/index caching logic.

Scope

• Exercise append writes through the Flink test harness for continuous-sort, Disruptor, and bounded in-memory buffers.
• Verify buffer selection, checkpoint flushing, written files, and resource cleanup.
• Cover dynamic bucket assignment for existing and new record keys, including checkpoint lifecycle behavior.
• Cover lazy record-level-index cache creation, checkpoint-aware eviction, dictionary encoding, and cache-size inference.
• Add focused coverage for BulkInsertWriterHelper error handling.

Acceptance criteria

• Each targeted class reaches at least 75% line coverage in Codecov.
• Tests run as unit tests and do not depend on Flink integration-test-only drivers.
• The pull request documents before/after per-class coverage and the validation command.