Fix @datastar_response typing for Django async views

[MarcusL11]

Summary

• Add @overload signatures to the Django datastar_response decorator so mypy can narrow the return type based on sync vs async input
• Use Coroutine[Any, Any, ...] instead of Awaitable[...] for the async overload, matching what django-stubs path() expects

Problem

The decorator currently has a single signature that returns a union type:

Callable[P, Awaitable[DatastarResponse] | DatastarResponse]

Mypy can't narrow this, so passing a decorated view to path() fails:

error: Argument 2 to "path" has incompatible type
  "Callable[[HttpRequest], Awaitable[DatastarResponse] | DatastarResponse]";
  expected "Callable[..., HttpResponseBase]"

This forces users to wrap every decorated view in cast():

path("my-view/", cast(ViewCallable, my_view))

Why Coroutine and not Awaitable

django-stubs accepts async views as Callable[..., Coroutine[Any, Any, HttpResponseBase]]. Since Coroutine is a subtype of Awaitable, using the broader Awaitable doesn't satisfy the narrower Coroutine expectation.

Test plan

• mypy passes for all 4 decorator variants (async/sync × value/generator) in path()
• Existing test_decorator_matrix.py Django tests pass (4/4)

[gazpachoking]

Is the Coroutine here correct? Doesn't that eliminate async generators, which should be valid?

@datastar_response
async def async_gen():
    yield SSE.patch_elements('<div id="aoeu"></div>')