Validate gem name against protected typos when creating pending trusted publisher

[Hemang360]

Adds GemTypo validation to OIDC::PendingTrustedPublisher on create, so names too similar to existing protected gems (e.g. type_kit vs typekit) are rejected upfront instead of at gem push time.

Closes #6273

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.49%. Comparing base (58b23ae) to head (f8994e4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6330      +/-   ##
==========================================
- Coverage   97.08%   94.49%   -2.59%     
==========================================
  Files         489      489              
  Lines       10386    10451      +65     
==========================================
- Hits        10083     9876     -207     
- Misses        303      575     +272     

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