fix: Materialize smallest dyn ints to use feature gate for i8/i16

[lukemanley]

Based on this comment: #20075 (comment)

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 79.52%. Comparing base (58f763a) to head (edcd377).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-core/src/utils/supertype.rs	83.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20108      +/-   ##
==========================================
- Coverage   79.53%   79.52%   -0.01%     
==========================================
  Files        1563     1563              
  Lines      217115   217117       +2     
  Branches     2464     2464              
==========================================
- Hits       172679   172671       -8     
- Misses      43876    43886      +10     
  Partials      560      560              

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