Fix ClientStateVar late mount default sync

[harsh21234i]

Fixes #6823

Summary

Fixes a global ClientStateVar rendering bug where a component that mounted while the shared value was non-default
could get stuck when the value was later pushed back to the default.

Changes

• Initialize global ClientStateVar local state from the shared refs mirror when available.
• Preserve the existing plain useState(default) behavior for non-global client state.
• Add regression coverage for:
  • global client state reading from the shared ref
  • global client state without a default using undefined
  • local client state not reading from shared refs
• Add a bugfix news fragment.

Testing

• uv run pytest tests/units/test_client_state.py -q
• uv run pytest tests/units/compiler/test_memoize_plugin.py -k client_state -q
• uv run ruff check reflex/experimental/client_state.py tests/units/test_client_state.py
• uv run ruff format --check reflex/experimental/client_state.py tests/units/test_client_state.py
• uv run pyright reflex/experimental/client_state.py tests/units/test_client_state.py
• uv run towncrier check --config pyproject.toml --dir . --compare-with origin/main

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[harsh21234i]

hey @adhami3310 can you go through this ?

[greptile-apps]

Greptile Summary

Fixes late-mounted global ClientStateVar synchronization by initializing local React state from the shared ref when available.

• Preserves explicit null shared values while falling back to the configured default only for undefined.
• Retains the original initializer behavior for non-global client state.
• Adds unit and browser regression coverage plus a bugfix news fragment.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains; the previously reported explicit-null issue is resolved because the initializer now falls back only when the shared value is undefined.

Important Files Changed

Filename	Overview
reflex/experimental/client_state.py	Initializes global client state from the shared ref using an undefined-only fallback, while preserving the local-state path.
tests/units/test_client_state.py	Covers shared-ref initialization, explicit null preservation, undefined fallback, and local-state isolation.
tests/integration/tests_playwright/test_client_state.py	Exercises synchronization when a component mounts after a global value changes and the value later returns to its default.
news/6823.bugfix.md	Documents the corrected late-mount synchronization behavior.

_{Reviews (3): Last reviewed commit: "Add ClientStateVar late mount integratio..." | Re-trigger Greptile}

[codspeed-hq]

Merging this PR will not alter performance

✅ 26 untouched benchmarks
⏩ 8 skipped benchmarks¹

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_{Comparing harsh21234i:fix/client-state-late-mount-default-sync (45c088e) with main (a08a061)}

Footnotes

1. 8 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[adhami3310]

can you add the repro as integration test

[harsh21234i]

Added the repro as a Playwright integration test in 45c088e. It follows the issue scenario: push the global
ClientStateVar to a non-default value, mount a second consumer while it is non-default, then push back to the default
and assert both consumers update.