Implement decentralized event handlers

[devin-ai-integration]

Implement Decentralized Event Handlers

This PR implements the ability to use decentralized event handlers in the Reflex framework, as described in ENG-5594.

Features

• Added support for defining event handlers outside of state classes using the @rx.event decorator
• Event handlers require an explicit state parameter that identifies the state they operate on
• Implemented automatic registration of decentralized event handlers in a global registry
• Updated event processing logic to handle decentralized event handlers
• Added integration tests to verify the functionality

Example Usage

@rx.event
def on_load(state: ChatState):
    """Event handler for loading the chat state."""
    state.field = 321

...

rx.button(on_mount=on_load)

Testing

• Added integration tests that verify decentralized event handlers work with on_mount and on_click events
• All existing tests pass, ensuring backward compatibility

Link to Devin run: https://app.devin.ai/sessions/5954950b5f294e9682d09edc224e891b
Requested by: khaleel@reflex.dev

[devin-ai-integration]

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[codspeed-hq]

CodSpeed Performance Report

Merging #5211 will improve performances by 4.82%

_{Comparing devin/1745972901-decentralized-event-handlers (6e98a68) with main (26f1a7b)}

Summary

⚡ 1 improvements
✅ 11 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_compile_stateful[_simple_page]	256.5 µs	244.7 µs	+4.82%

[adhami3310]

import reflex as rx


class State(rx.State):
    """The app state."""

    value: int = 32


@rx.event
def set_values(state: State, value: str):
    state.value = int(value)
    print(f"Value set to {state.value}")


def index() -> rx.Component:
    return rx.button(
        "Click me",
        State.value,
        on_click=set_values("100"),
    )


app = rx.App()
app.add_page(index)

that needs to work but:

  File "/home/khaleel/Projects/reflex/reflex/examples/demo/demo/demo.py", line 20, in index
    on_click=set_values("100"),
             ~~~~~~~~~~^^^^^^^
TypeError: set_values() missing 1 required positional argument: 'value'

[amsraman]

wrong directory :(