feat: Add env var to ignore file cache allocate error

[nameexhaustion]

Adds a flag POLARS_IGNORE_FILE_CACHE_ALLOCATE_ERROR to ignore any allocation errors from the std::fs::File::allocate call. May help with:

• scan_csv() in container fails with disk space error (e.g. AWS lambda, or container) #17946

There is a chance this allocate is incorrectly failing. I've also adjusted the print to include the IO error itself.

[codecov]

Codecov Report

Attention: Patch coverage is 41.66667% with 14 lines in your changes missing coverage. Please review.

Project coverage is 79.12%. Comparing base (6e4d717) to head (99d57b6).
Report is 32 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-io/src/file_cache/entry.rs	41.66%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20356      +/-   ##
==========================================
- Coverage   79.13%   79.12%   -0.01%     
==========================================
  Files        1572     1572              
  Lines      219839   219859      +20     
  Branches     2462     2462              
==========================================
- Hits       173970   173966       -4     
- Misses      45301    45325      +24     
  Partials      568      568              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.