Fix buffered upload handler not cancelling on client disconnect (#6307)

* Fix buffered upload handler not cancelling on client disconnect

When a client disconnects during a buffered file upload, the enqueued
event handler was not being cancelled, leaving orphaned tasks. Add
disconnect detection to _UploadStreamingResponse (via ASGI 2.4 receive
watcher and ClientDisconnect handling) and propagate an on_task_future
callback through enqueue_stream_delta so the upload path can cancel the
EventFuture when the connection drops.

* Update packages/reflex-components-core/src/reflex_components_core/core/_upload.py

Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>

* Update packages/reflex-components-core/src/reflex_components_core/core/_upload.py

Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>

* Generalize disconnect-cancel for streaming event responses

Move disconnect detection out of upload-specific code into a reusable
DisconnectAwareStreamingResponse in reflex-base. Drop the on_task_future
callback from enqueue_stream_delta — the generator"s finally block already
cancels the task future when the body iterator is closed on disconnect.

* Add pre-2.4 streaming response disconnect test, remove bogus event processor tests

Add test coverage for the pre-2.4 ASGI disconnect path in
DisconnectAwareStreamingResponse.

Remove test_stream_delta_aclose_cancels_in_flight_event and
test_stream_delta_consumer_task_cancellation_cancels_in_flight_event — they
passed with or without the task_future.cancel() fix because _on_future_done
already cascades cancellation from the EventFuture to the handler task.

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Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>