feat!: static handler configuration

[provinzkraut]

Basic idea is this: Instead of mutating handlers in place, treat them as static configuration objects. Once the handler tree has been collected, reduce all the layers into a single handler by recursively merging them.

Why? The idea is that this design is much less error prone and easier to follow (e.g. a lot of things currently need to happen in a specific order, otherwise they'll break). It also supports a pattern we're using in a few places now (websocket listeners + stream), where you essentially want to replace the handler with a new one. Another benefit is that this can get rid of all the deep copying we're currently doing :)

Goals

• "Merge" handlers instead of setting ownership layers
• Move all routing functionality from Router to Litestar instance
• Remove (deprecate) unnecessary resolve_ methods
• Remove ownership layer API completely
• Remove deepcopying of route handlers during registration
• Extract common handler config into config objects (dataclasses), so they're easier to merge / manipulate
• Make route handlers purely static by removing all in-place mutation

---

I've decided to strike the last two goals, as it would make a migration path forward needlessly complex. With the removal of the now deprecated functionality in 4.0, this will be a lot easier to achieve, and we're still in a much better place with this than before.

[codecov]

Codecov Report

Attention: Patch coverage is 98.27957% with 8 lines in your changes missing coverage. Please review.

Project coverage is 98.26%. Comparing base (a2afa51) to head (04c3c2a).
Report is 1 commits behind head on v3.0.

Files with missing lines	Patch %	Lines
litestar/handlers/base.py	95.31%	4 Missing and 2 partials ⚠️
litestar/app.py	98.66%	0 Missing and 1 partial ⚠️
...estar/handlers/websocket_handlers/route_handler.py	95.65%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             v3.0    #3900      +/-   ##
==========================================
- Coverage   98.30%   98.26%   -0.05%     
==========================================
  Files         342      342              
  Lines       15370    15350      -20     
  Branches     1694     1662      -32     
==========================================
- Hits        15109    15083      -26     
- Misses        129      133       +4     
- Partials      132      134       +2     

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[cofin]

I don't have much to add other than what we've already discussed here and on discord. Great work on this.

[sonarqubecloud]

Quality Gate failed

Failed conditions
7 New Major Issues (required ≤ 2)
7 New Code Smells (required ≤ 3)

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