Detect asset-level deletion in backfill staleness checks

[rakhiagr]

Summary

• When an entity is deleted via deleteAll()/softDeleteAsset(), deleted_ts = NOW() is set on the entity row. batchGetUnion() always filters by AND deleted_ts IS NULL, making asset-deleted entities invisible — indistinguishable from "never existed" — so shouldBackfill() unconditionally allows stale backfills, resurrecting deleted entities.
• Fix: when batchGetUnion() returns empty in the new-schema path, perform a fallback query to check deleted_ts. If the entity is asset-deleted, return a synthetic soft-deleted EbeanMetadataAspect with the deletion timestamp, so shouldBackfill() correctly compares emitTime > deletionTimestamp and rejects stale events.

RFC: https://docs.google.com/document/d/1zOKrxbm_zGxTpMwJ7Wer7owY-YcI0nXDvY6DWRGXj0M/edit?tab=t.0#heading=h.nwcowzxl6in9

Depends on: #602 (Gap 1 + Gap 2 fix)

Backward Compatibility

• The fallback query only runs when batchGetUnion() returns empty in the NEW_SCHEMA_ONLY path — no impact on existing read flows
• Entities that are not asset-deleted are unaffected (fallback returns null, existing behavior preserved)

Testing Done

Unit tests added:

• testGetAssetDeletionTimestampSql — SQL generation for normal mode
• testGetAssetDeletionTimestampSqlTestMode — SQL generation for test mode
• testGetAssetDeletionTimestampReturnsNullForActiveEntity — active entity returns null
• testGetAssetDeletionTimestampReturnsNullForNonExistentEntity - non-existent entity returns null
• testGetAssetDeletionTimestampReturnsTimestampForDeletedEntity — asset-deleted entity returns timestamp
• testGetAssetDeletionTimestampInvisibleToBatchGetUnion — confirms batchGetUnion returns empty but getAssetDeletionTimestamp finds the deleted entity

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Docs related to the changes have been added/updated (if applicable)