branch-4.1: [fix](case) fix mow compaction cases on multi-replica cluster

[shuke987]

What problem does this PR solve?

The three nonConcurrent MOW compaction regression cases

• test_mow_compaction_and_read_stale
• test_mow_compaction_and_schema_change
• test_mow_compaction_agg_and_remove_pre_delete_bitmap

asserted assertEquals(1, tablets.size()) right after show tablets.
show tablets returns one row per replica, so on a cluster that
materializes multiple replicas for the single bucket (the branch-4.1
NonConcurrent pipeline creates 3 replicas even when the table requests 1),
the assertion fails at table setup:

expected: <1> but was: <3>

The 3 rows share the same TabletId (one bucket, three replicas), e.g.:

TabletId:1781362443532 ReplicaId:...533 BackendId:...351
TabletId:1781362443532 ReplicaId:...534 BackendId:...348
TabletId:1781362443532 ReplicaId:...535 BackendId:...349

What does this PR do?

Assert the real invariant instead — exactly one distinct tablet (BUCKETS 1) —
by de-duplicating on TabletId, which is independent of replica count:

assertEquals(1, tablets.collect { it.TabletId }.unique().size())

The cases already operate on a single replica via tablets[0], so the
downstream compaction / delete-bitmap inspection is unaffected.

Related to #58662, which relaxed the same check to
tablets.size() >= 1; this uses a stricter distinct-tablet assertion. The
same improvement will be proposed on master so the branches reconverge.

Release note

None

Check List

• Test
  • Regression test (test-only change)

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[shuke987]

run buildall