fix(python): retain returned effect cleanups

[mangod12]

Setup-returned cleanup callbacks are now kept separate from the explicit cleanup= callback, so rerenders do not erase them. Returned cleanup runs before a dependency-triggered setup and on unmount; unmount also queues cancellation of a pending async setup task.

Tests: 11 focused Python tests pass; Ruff check and format pass.

AI-assisted implementation; reviewed and tested. Fixes #6963.

Summary by Sourcery

Preserve returned effect cleanups throughout the effect lifecycle and cancel pending asynchronous setups during unmount.

Bug Fixes:

• Preserve effect setup-returned cleanup callbacks across rerenders and execute them before dependency-triggered setup or on unmount.
• Ensure unmount schedules cleanup when an effect setup task is still pending, allowing the task to be cancelled.

Enhancements:

• Separate setup-returned cleanup callbacks from explicit cleanup callbacks and track effect lifecycle work consistently.

Tests:

• Add focused tests covering cleanup retention, rerender and unmount behavior, and cancellation of pending asynchronous setups.

[CLAassistant]

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[sourcery-ai]

Hey - I've reviewed your changes and they look great!

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