refactor(rust): Change fn schema to fn collect_schema

[ritchie46]

Rename schema to collect_schema to indicate we are doing work.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.22%. Comparing base (5d874b1) to head (7ce2517).
Report is 48 commits behind head on main.

Files	Patch %	Lines
crates/polars-python/src/lazygroupby.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18261      +/-   ##
==========================================
- Coverage   80.25%   80.22%   -0.03%     
==========================================
  Files        1500     1500              
  Lines      198906   198906              
  Branches     2837     2837              
==========================================
- Hits       159641   159582      -59     
- Misses      38738    38797      +59     
  Partials      527      527              

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