fix(dto/msgspec_dto): Fix KeyError when ClassVar exists on msgspec dto struct

[Peopl3s]

Description

Fix KeyError when ClassVar exists on msgspec dto struct

Closes #4665

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📚 Documentation preview 📚: https://litestar-org.github.io/litestar-docs-preview/4669

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.33%. Comparing base (1a133ca) to head (1f30b8f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4669   +/-   ##
=======================================
  Coverage   67.33%   67.33%           
=======================================
  Files         292      292           
  Lines       14939    14941    +2     
  Branches     1675     1676    +1     
=======================================
+ Hits        10059    10061    +2     
  Misses       4743     4743           
  Partials      137      137           

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[provinzkraut]

Thanks!