Close receiver when stopping LatestValueCache

[shsms]

No description provided.

[Copilot]

Pull request overview

This PR updates LatestValueCache to take ownership of its Receiver and ensure the receiver is closed when the cache is stopped, preventing unused receivers from remaining attached to channels.

Changes:

• Document that LatestValueCache takes ownership of the receiver and will close it on stop.
• Call self._receiver.close() from LatestValueCache.stop().

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[Copilot]

stop() now closes the underlying receiver, but stop()'s docstring still only says "Stop the cache". Please update the method docstring to mention that it also closes the owned receiver, so callers don't accidentally expect to keep using it after stopping.

Suggested change

	"""Stop the cache."""
	"""Stop the cache and close the owned receiver.
	After this method is called, the underlying receiver is closed and
	must not be used anymore.
	"""

[Copilot]

Since this is a behavior/ownership change, consider also updating the module/package-level documentation for LatestValueCache (e.g., the module docstring and/or the entry in frequenz.channels.__init__ docs) so users reading the top-level docs learn that stop() closes the receiver.