Pwmj metrics accounting

[SubhamSinghal]

Which issue does this PR close?

Part of #17427.

Rationale for this change

PiecewiseMergeJoinExec's metrics accounting had several gaps left over from earlier PRs in the epic:

• The classic-join stream (Left/Right/Full/Inner) never routed poll_next through BaselineMetrics::record_poll, so output_rows, output_bytes, and output_batches stayed at 0 in EXPLAIN ANALYZE regardless of how many rows the join actually produced.
• join_time was never measured on either the classic or existence-join stream — only build_time was timed, so elapsed_compute understated total operator cost for large probe sides.
• probe_hit_rate/avg_fanout exist on the shared BuildProbeJoinMetrics struct (populated by HashJoinExec) but PWMJ never populated them, always showing N/A (0/0).

What changes are included in this PR?

• ClassicPWMJStream::poll_next now calls self.join_metrics.baseline.record_poll(poll), matching the existence-join stream.
• join_time is timed around the actual comparison work: resolve_classic_join and the ProcessUnmatched bitmap/take pass on the classic path; extreme_key + mark_matched_buffered_rows on the existence path.
• probe_hit_rate/avg_fanout are populated for classic join, using the range size (buffered_len - buffer_idx) already computed at each match as the fanout.
• probe_hit_rate is populated for existence join: a streamed batch counts as a hit if its extreme key lowers the shared watermark, a miss otherwise. avg_fanout is intentionally left unset for existence join — its watermark-based semantics don't have a natural per-row fanout equivalent.
• Added a metrics-focused test to each stream module: classic_join::tests::inner_join_records_output_and_probe_metrics and existence_join::tests::probe_hit_rate_counts_batches_that_advance_the_watermark, both hand-derived and verified against actual runs.

Are these changes tested?

Yes — two new unit tests

Are there any user-facing changes?

EXPLAIN ANALYZE on a PiecewiseMergeJoinExec plan now reports accurate output_rows/output_bytes/output_batches/join_time/probe_hit_rate/avg_fanout instead of zeros/N/A. No API or behavior change to query results.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.31776% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.61%. Comparing base (35f58f5) to head (d24e66a).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...n/src/joins/piecewise_merge_join/existence_join.rs	88.03%	4 Missing and 10 partials ⚠️
...lan/src/joins/piecewise_merge_join/classic_join.rs	88.65%	0 Missing and 11 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #24956    +/-   ##
========================================
  Coverage   81.61%   81.61%            
========================================
  Files        1124     1124            
  Lines      411978   412288   +310     
  Branches   411978   412288   +310     
========================================
+ Hits       336236   336499   +263     
- Misses      55936    55955    +19     
- Partials    19806    19834    +28     

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