fix: Ensure same fmt in Series/AnyValue to string cast

[ritchie46]

fixes #18981

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.85%. Comparing base (3904774) to head (aa2d9a5).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-core/src/datatypes/any_value.rs	66.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18982      +/-   ##
==========================================
- Coverage   79.86%   79.85%   -0.02%     
==========================================
  Files        1523     1523              
  Lines      207504   207510       +6     
  Branches     2904     2904              
==========================================
- Hits       165720   165703      -17     
- Misses      41237    41260      +23     
  Partials      547      547              

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