fix: Ensure 'CachedSchema' doesn't get synced between plans

[ritchie46]

fixes #15651

[codspeed-hq]

CodSpeed Performance Report

Merging #15661 will not alter performance

_{Comparing cache (cfeaec1) with main (f992a7a)}

Summary

✅ 22 untouched benchmarks

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 81.31%. Comparing base (99ab9c0) to head (cfeaec1).
Report is 7 commits behind head on main.

Files	Patch %	Lines
...s/polars-plan/src/logical_plan/functions/schema.rs	70.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15661      +/-   ##
==========================================
- Coverage   81.31%   81.31%   -0.01%     
==========================================
  Files        1373     1373              
  Lines      176254   176271      +17     
  Branches     2544     2544              
==========================================
+ Hits       143320   143330      +10     
- Misses      32452    32457       +5     
- Partials      482      484       +2     

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