perf: OOC sort improvements

[ritchie46]

Don't force spilling before going to source phase. And try to keep in memory when we can.

[codecov]

Codecov Report

Attention: Patch coverage is 68.60465% with 27 lines in your changes are missing coverage. Please review.

Project coverage is 80.94%. Comparing base (8c2eca9) to head (e30f1ec).
Report is 5 commits behind head on main.

❗ Current head e30f1ec differs from pull request most recent head bc9f37d. Consider uploading reports for the commit bc9f37d to get more accurate results

Files	Patch %	Lines
...tes/polars-pipe/src/executors/sinks/sort/source.rs	50.00%	27 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14994      +/-   ##
==========================================
- Coverage   80.99%   80.94%   -0.05%     
==========================================
  Files        1333     1333              
  Lines      173174   173205      +31     
  Branches     2458     2460       +2     
==========================================
- Hits       140265   140208      -57     
- Misses      32442    32529      +87     
- Partials      467      468       +1     

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