refactor: keep iterator on the main thread

[winstxnhdw]

Description

To prevent cross-thread reference errors with Stream iterators, we keep iterators on the main context and listen for a disconnect asynchronously instead of vice versa.

I have properly split the changes into 3 separate commits for better diffs.

Other changes

• Reuse stream_event dictionary to avoid recreating the dictionary on every iteration.
• Remove redundant type checks, e.g. AsyncIterable covers AsyncIterator too.
• Some miscellaneous inlining and using terse expressions like chain(self.cookies, cookies or ())

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.84%. Comparing base (08e8dad) to head (83b668e).

Files with missing lines	Patch %	Lines
litestar/response/streaming.py	95.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4432   +/-   ##
=======================================
  Coverage   97.83%   97.84%           
=======================================
  Files         296      296           
  Lines       15286    15287    +1     
  Branches     1711     1712    +1     
=======================================
+ Hits        14955    14957    +2     
  Misses        189      189           
+ Partials      142      141    -1     

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[provinzkraut]

Can you explain this change a bit more? Where is something handed off to a different thread?

[github-actions]

Documentation preview will be available shortly at https://litestar-org.github.io/litestar-docs-preview/4432

[winstxnhdw]

Can you explain this change a bit more? Where is something handed off to a different thread?

Originally, we start self._stream in another task like so.

async with create_task_group() as task_group:
    task_group.start_soon(partial(self._stream, send))
    await self._listen_for_disconnect(cancel_scope=task_group.cancel_scope, receive=receive)

If a user, who holds the reference to the iterator, tries to do something with it, Python raises a ValueError: generator already executing error. This only happens with a sync generator, presumably because we use sync_to_thread which actually hands it off to a different thread, and the combination of the stream being in a different context and iterating in a separate thread somehow leads to this issue.