fix: ensure BaseRouteHandler.on_registration is only called once per handler

[provinzkraut]

Fix an issue that would lead to BaseRouteHandler.on_registration being called more than once if a handler instance was registered for multiple paths (i.e. @get(["/a", "/b"])). This fix only targets v3 even though the behaviour also exists in v2. In v2 this is much harder to fix and uncertain which side effects it might introduce in downstream apps due to the mutable nature of the handlers pre #3900.

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📚 Documentation preview 📚: https://litestar-org.github.io/litestar-docs-preview/4738

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 67.38%. Comparing base (d63f917) to head (0dbccc7).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
litestar/app.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4738   +/-   ##
=======================================
  Coverage   67.37%   67.38%           
=======================================
  Files         292      292           
  Lines       14996    14999    +3     
  Branches     1684     1685    +1     
=======================================
+ Hits        10104    10107    +3     
  Misses       4756     4756           
  Partials      136      136           

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