fix(dojo-lang): use PluginDiagnostic instead of panic for model keys verification

[glihm]

@kariy suggested the use of PluginDiagnostics, which enhance the debug experience:

Also, the error handling is now done by model.rs instead of being done inside the Introspect handling.

Happy to have feedback on the wording if it needs to be adjusted. 👍

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (e351f0f) 64.27% compared to head (c2dd18c) 64.27%.

❗ Current head c2dd18c differs from pull request most recent head 04e1847. Consider uploading reports for the commit 04e1847 to get more accurate results

Files	Patch %	Lines
crates/dojo-lang/src/model.rs	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1327      +/-   ##
==========================================
- Coverage   64.27%   64.27%   -0.01%     
==========================================
  Files         229      229              
  Lines       19711    19708       -3     
==========================================
- Hits        12669    12667       -2     
+ Misses       7042     7041       -1     

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[kariy]

i forgot to mention in the initial review, but let's include a test for the new behaviour in here also. Then, we're good to merge.

[glihm]

Sure thing. Sorry for not putting them at the first round.. Thank you again. 👍