Currently Zed only displays agent notifications (e.g. when the agent
completes a task) if the user has switched apps and Zed is not in the
foreground. This adds PR supports the scenario where the agent finishes
a long-running task and the user is busy coding within Zed on something
else.

Releases Note:

- If agent notifications are turned on, they will now also be displayed
when the agent panel is hidden, in complement to them showing when the
Zed window is in the background.

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Co-authored-by: Danilo Leal <daniloleal09@gmail.com>