test(rust): Make assumption about column name to index conversion having occurred explicit

[mickvangelderen]

Alternatively, we could do the conversion again if it has not occurred yet.

The column name to indices conversion happens in multiple places in the code base.

In those places we use only the column names if provided and ignore the column indices (projection) without warning. We could take the union instead, print a warning about ignoring the indices when the names are provided, or do nothing.

Depending on this choice, we may be able to represent the projection with a better data structure and deduplicate the conversion code.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 81.01%. Comparing base (3cf4897) to head (335cf05).
Report is 1 commits behind head on main.

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Files	Patch %	Lines
crates/polars-io/src/ipc/ipc_file.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14894   +/-   ##
=======================================
  Coverage   81.01%   81.01%           
=======================================
  Files        1331     1331           
  Lines      172773   172776    +3     
  Branches     2455     2455           
=======================================
+ Hits       139966   139972    +6     
+ Misses      32339    32337    -2     
+ Partials      468      467    -1     

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