… storage TopN fix

### What problem does this PR solve?

Issue Number: None

Related PR: apache#15663

Problem Summary:

Follow-up to the previous commit, from its review:

- The regression test also covers `ORDER BY k1 DESC LIMIT 10` and a cluster key table read by
  `ORDER BY <cluster key prefix> LIMIT 10`, which lost rows the same way (on the unfixed BE they
  return k1 = 19..10 and v = 0..9 once each), checks with explain that the TopN reaches the scan,
  and adds a table whose two segments hold the same full keys: the storage TopN no longer drops
  rows itself, and the delete bitmap of the merge-on-write table hides the older row of each key,
  so every key comes back once with the newest value. The needless `SET batch_size = 4064` is
  gone; the `.out` is regenerated on the fixed build.
- The compare contract error names the condition it checks, `is_unique with
  read_orderby_key_columns set`, and the unit test matches it; the comments say rows equal on the
  ORDER BY columns may be different keys, not that they are.
- The rowset writing helper of BetaRowsetTest stops at the first failed step and prints its status.

Corrections to the previous commit message: the bug came with apache#15663 (2023-02-06), which extended
the storage TopN to merge-on-write tablets; apache#10694 had introduced the merge by the ORDER BY columns
for DUP tablets only, which drop nothing. The query returns wrong rows: the rows equal on the
prefix are lost and rows of later prefixes take their place, so it is short only when too few rows
remain. Every branch since apache#15663 has the bug (branch-2.0 to branch-4.2 by reading the code); a
backport takes the BetaRowsetReader change only, since the release branches have no compare
contract check and `enable_segment_limit_pushdown` exists only on branch-4.2 and master.

### Release note

None

### Check List (For Author)

- Test: Unit Test / Regression test
    - BetaRowsetTest and VGenericIteratorsTest: 24 pass.
    - fault_injection_p0/test_mow_topn_key_prefix_overlapping_segments passes on the fixed BE; on
      an unfixed BE the prefix, DESC and cluster key queries return the wrong rows above, and the
      same-key table returns one row per key with the newest value.
    - clang-format 16 and clang-tidy clean on the changed lines; build hygiene passes.
- Behavior changed: No
- Does this need documentation: No

Co-Authored-By: Claude Opus 5.5 <[email protected]>