fix: record join_time and output metrics in PiecewiseMergeJoin

[buraksenn]

Which issue does this PR close?

• Closes N/A

Rationale for this change

EXPLAIN ANALYZE on a PiecewiseMergeJoin always reports join_time=0 and, for classic joins, output_rows=0 / output_bytes=0.0 B, so PWMJ plans can't be profiled like other joins.

What changes are included in this PR?

• Classic stream: time probe-phase work under join_time (started only after the streamed input's poll returns Ready) and record output metrics via record_poll
• Existence stream: same join_time coverage for the scan and final pass
• elapsed_compute picks both up via the existing BuildProbeJoinMetrics drop

Are these changes tested?

Yes. Classic tests now assert assert_join_metrics!, the existence final-pass test pins join_time > 0, and a new test verifies join_time excludes streamed-input wait. explain_analyze.slt shows the real output_bytes.

Are there any user-facing changes?

No (PWMJ is experimental and off by default).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.65363% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.45%. Comparing base (e2e94ae) to head (f08cd43).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...lan/src/joins/piecewise_merge_join/classic_join.rs	75.90%	28 Missing and 12 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24689      +/-   ##
==========================================
- Coverage   81.45%   81.45%   -0.01%     
==========================================
  Files        1119     1119              
  Lines      400030   400182     +152     
  Branches   400030   400182     +152     
==========================================
+ Hits       325850   325949      +99     
- Misses      55144    55179      +35     
- Partials    19036    19054      +18     

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[jayzhan211]

Thanks @buraksenn, LGTM