Fix client state setter event var args

[wantosure]

Fixes #5741.

ClientStateVar.set_value() was using the rendered value expression to build the wrapper function argument. That works for plain event vars, but breaks for optional-chain expressions like _event?.clientX because the generated handler ends up with _event?.clientX as the argument name:

((_event?.clientX) => ...)

This patch keeps the setter call value as-is, but derives the wrapper argument from the leading event identifier instead, so the generated function is valid:

((_event) => ...(_event?.clientX))

Added a regression test for the optional-chain client-state setter case.

Tested with:

python -m pytest tests\units\test_event.py -q
python -m pytest tests\units\test_event.py::test_call_function_client_state_setter_accepts_optional_chain_var tests\units\compiler\test_memoize_plugin.py::test_client_state_setter_in_call_function_event_imports_refs -q
python -m ruff check reflex\experimental\client_state.py tests\units\test_event.py

Pytest warned that it could not write .pytest_cache in my Windows workspace, but the tests passed.

[greptile-apps]

Greptile Summary

This PR updates client-state setters to derive valid wrapper argument names from event-derived expressions while preserving the complete setter value expression.

• Adds Unicode-aware JavaScript identifier parsing for event arguments.
• Adds regression coverage for optional-chain and Unicode event variables.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
reflex/experimental/client_state.py	Replaces expression-based wrapper parameters with leading event-identifier extraction so optional-chain setter expressions generate valid JavaScript.
tests/units/test_event.py	Adds focused serialization tests for optional-chain and Unicode client-state setter event expressions.

_{Reviews (2): Last reviewed commit: "Handle unicode event argument names" | Re-trigger Greptile}

[cubic-dev-ai]

All reported issues were addressed across 2 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[wantosure]

Thanks, this was a valid catch.

The old regex only handled ASCII identifier characters, so an event expression such as _événement?.clientX would bind _ as the callback arg and then reference _événement inside the body.

I replaced that with a small Unicode-aware JS identifier scanner and added a regression test for the non-ASCII event arg case.

Verified with:

• python -m pytest tests\units\test_event.py -q
• python -m ruff check reflex\experimental\client_state.py tests\units\test_event.py