Fixes https://github.com/zed-industries/zed/issues/9340

The flickering was caused by the pane trying to restore focus on a
`FocusHandle` that wasn't being rendered anymore. This commit uses the
new `WeakFocusHandle` to avoid retaining a reference to focus handles
that don't exist anymore.

Release Notes:

- Fixed a bug that caused flickering when interacting with the language
server logs
([#9340](https://github.com/zed-industries/zed/issues/9340)).