[flink] Delta Join additional IT tests and docs improvement

[fresh-borzoni]

Purpose

Linked issue: close #2231

Add comprehensive test coverage for Delta Join feature in Flink 2.2 and improve documentation.

Brief change log

• Added tests for normal PK tables with table.delete.behavior=IGNORE (not just first_row merge engine)
• Added tests for joining on bucket key (not just full primary key)
• Added tests verifying LEFT/RIGHT/FULL OUTER joins fail with appropriate validation error
• Added test verifying cascade joins are not supported
• Updated documentation for Flink 2.2 Delta Join with new supported features and limitations

Tests

• testDeltaJoinWithPrimaryKeyTableNoDeletes - normal PK table with delete.behavior=IGNORE
• testDeltaJoinOnBucketKey - join on bucket key only
• testDeltaJoinFailsWhenFilterOnNonUpsertKeys - filter on non-upsert-key columns fails
• testDeltaJoinOnBucketKey - join on bucket key only (not full PK)
• testDeltaJoinFailsWhenSourceHasDelete - source with DELETE records fails
• testDeltaJoinFailsWhenJoinKeyNotContainIndex - join key not containing index fail
• testDeltaJoinFailsWithLeftJoin - LEFT JOIN wouldn't be converted to DeltaJoin
• testDeltaJoinFailsWithRightJoin - RIGHT JOIN wouldn't be converted to DeltaJoin
• testDeltaJoinFailsWithFullOuterJoin - FULL OUTER JOIN wouldn't be converted to DeltaJoin
• testDeltaJoinFailsWithCascadeJoin - cascade join wouldn't be converted to DeltaJoin

API and Format

No

Documentation

Yes - updated docs/engine-flink/delta-joins.md in Flink 2.2 part.

[fresh-borzoni]

@xuyangzhong @wuchong
While studying the Flink 2.2 changes related to Delta Join, I noticed this issue. I've added some tests and improved the documentation, feel free to use the changes if you find them useful.