fix: drain PiecewiseMergeJoin output before state transitions

[buraksenn]

Which issue does this PR close?

• Closes PiecewiseMergeJoin changes state before draining its output coalescer: the Left/Full unmatched pass repeats forever, queued Right/Full rows can be dropped #24297

Rationale for this change

ClassicPWMJStream transitions phases while its output BatchCoalescer still holds completed batches. The Left/Full unmatched pass falls back into its producer when the drained queue is an exact multiple of batch_size — emitting duplicate rows forever — and queued unmatched Right/Full batches are dropped at terminal transitions. A zero-row placeholder batch also escapes as stream output on empty results.

What changes are included in this PR?

• Add BatchProcessState::next_drained_batch() (finish_buffered_batch + next_completed_batch); None guarantees the coalescer is empty
• process_stream_batch and process_unmatched_buffered_batch drain one batch per poll and transition only after a confirmed-empty pop, so a finished producer can never re-run and no queued batch is lost
• Scan completion returns StatefulStreamResult::Continue instead of the empty placeholder batch; empty joins now yield no batches

Are these changes tested?

Yes. The existing unmatched Left/Right tests now run with batch_size = 1 (the Left one bounded with take(6) so the old duplicate loop fails as a snapshot mismatch instead of hanging) — both fail on main and pass here. The empty-join tests assert no batches are emitted.

Are there any user-facing changes?

No (PWMJ is experimental and off by default).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 74.19355% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.17%. Comparing base (e76f1af) to head (90f333b).

Files with missing lines	Patch %	Lines
...lan/src/joins/piecewise_merge_join/classic_join.rs	74.19%	0 Missing and 8 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #24298   +/-   ##
=======================================
  Coverage   81.17%   81.17%           
=======================================
  Files        1109     1109           
  Lines      388033   388001   -32     
  Branches   388033   388001   -32     
=======================================
- Hits       314992   314968   -24     
+ Misses      54501    54489   -12     
- Partials    18540    18544    +4     

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