fix(jwt): report correct claim name for invalid iss

[janmrow]

JWTClaimsRegistry.validate_iss currently reports "str" instead of "iss" when the iss claim has an invalid type.

This PR fixes the reported claim name and error description. Making it consistent with the sub and aud validators.

The existing iss validation test now also checks the reported claim name and description.

Tests on the local machine:

• pytest tests/ -q
• ruff format --check src/joserfc/_rfc7519/claims.py tests/jwt/test_claims.py
• mypy src/joserfc/_rfc7519/claims.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (357c319) to head (eec6d5e).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #105   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files           47        47           
  Lines         2945      2945           
  Branches       350       350           
=========================================
  Hits          2945      2945           

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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