fix(typing): allow ImmutableState instance to be passed to Litestar/AppConfig

[osauldmy]

Description

• Hi. I wanted to use custom child of ImmutableState to be passed to Litestar instance, but mypy warned me I can't really do it, so instead of type-ignoring it, I created this PR.
• Maybe a bit cleaner solution (but not sure if it'll need more work) is to swap inheritance, so ImmutableState inherits from State instead, but I suppose it'll still require isinstance asserts I added in tests to satisfy type-checkers.

Closes

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📚 Documentation preview 📚: https://litestar-org.github.io/litestar-docs-preview/4653

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.32%. Comparing base (5875eac) to head (6470192).

Files with missing lines	Patch %	Lines
litestar/config/app.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4653   +/-   ##
=======================================
  Coverage   67.32%   67.32%           
=======================================
  Files         292      292           
  Lines       14933    14933           
  Branches     1673     1673           
=======================================
  Hits        10054    10054           
  Misses       4743     4743           
  Partials      136      136           

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