[lake/tiering] add table dropped handling and nullable write results

[beryllw]

Purpose

Linked issue: close #2498

This PR improves Lake Tiering's handling of dropped tables and adds cancellation support for tiering operations.

Brief change log

Dropped Table Handling

• When a table is dropped during active tiering, the TieringSplitReader now gracefully completes all
  in-progress splits with empty results instead of failing
• Added handleTableDropped() to mark dropped tables and trigger force completion
• Dropped tables are properly cleaned up from LakeTableTieringManager to prevent resource leaks

Cancellation Support

• Added cancelled flag to TableBucketWriteResult to distinguish between normal and cancelled tiering rounds
• Committers skip commit processing for cancelled results and report back to the coordinator
• Lake writers are closed without completing when tiering is cancelled, discarding uncommitted data

Tests

API and Format

Documentation