[flink] fix compact_database parameter mode default value

[sd4324530]

Purpose

Linked issue: close #6934
fix compact_database parameter mode default value

Tests

API and Format

Documentation

[sd4324530]

@JingsongLi
I just fixed the default value of the 'mode' parameter, these failed checks. I don't understand why.
Please take a look.

[sd4324530]

@JingsongLi
All checks have passed.
Could you please take a look?

[JingsongLi]

Clean bug fix. The change makes sense - changing the default compact_database mode from DIVIDED to COMBINED.

A few notes:

1. Breaking change: This changes the default behavior. Users who relied on the implicit DIVIDED default will now get COMBINED behavior unless they explicitly specify --mode divided. This should be documented in release notes.

2. Test update: Good that the test is updated to explicitly pass --mode divided where DIVIDED behavior is expected, rather than relying on the default.

3. Linked issue #6934: The fix aligns with the documented default in the documentation/user-facing docs.

LGTM with the caveat that this is technically a behavior change and should be called out in release notes.

[JingsongLi]

re-trigger tests

[JingsongLi]

This will cause compatibility issues, and we need to update the documentation. If you're willing, you can submit another PR for the document.