perf: Rechunk only once during join probe gather

[orlp]

The gather of the payload after finding matches does a rechunk internally. We now rechunk eagerly (as long as 1 match is found) to avoid the repeated rechunk. This sped up query 16 in PDS-H by ~2x.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 37 lines in your changes missing coverage. Please review.

Project coverage is 79.26%. Comparing base (3e70e5a) to head (2b0c7d2).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-stream/src/nodes/joins/equi_join.rs	0.00%	37 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21072      +/-   ##
==========================================
- Coverage   79.27%   79.26%   -0.01%     
==========================================
  Files        1585     1586       +1     
  Lines      225622   225648      +26     
  Branches     2588     2588              
==========================================
+ Hits       178865   178869       +4     
- Misses      46167    46189      +22     
  Partials      590      590              

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