fix: add better message for u256 being used as a key

[glihm]

More descriptive message to ensure the user knows why u256, which seem a core type as felt252, is not supported as a key into models.

[codecov]

Codecov Report

Attention: Patch coverage is 53.84615% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 66.95%. Comparing base (09762ad) to head (d85512d).

Files	Patch %	Lines
crates/dojo-lang/src/model.rs	50.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1769      +/-   ##
==========================================
- Coverage   66.96%   66.95%   -0.02%     
==========================================
  Files         305      305              
  Lines       33640    33653      +13     
==========================================
+ Hits        22528    22531       +3     
- Misses      11112    11122      +10     

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