feat: add create_reload_checker() for detecting hot reloads in long-running tasks

[maartenbreddels]

Summary

• Adds solara.create_reload_checker() which returns a function that detects when a hot reload has occurred
• Useful for long-running tasks (threads, async tasks) that need to exit gracefully after a reload to avoid issues with stale references to old module state
• Added documentation in the reloading reference page and in the task decorator docstring

Usage

import solara

did_reload = solara.create_reload_checker()

while not did_reload():
    # do work...
    await asyncio.sleep(1)
# Task exits gracefully after hot reload

Test plan

• Added unit test for create_reload_checker()
• All existing reload tests pass

Closes #1103

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