Closes https://github.com/zed-industries/zed/issues/33827

After #33644 was merged, we would not start MCP servers coming from
extensions correctly anymore. The optimization uncovered a bug in the
implementation of `ContextServerDescriptorRegistry`, because we never
called `cx.notify()` when adding/removing context servers.
`ContextServerStore` listens for these events, and before #33644 this
was just working because of aace condition.

Release Notes:

- agent: Fixed bug that prevented MCP servers to appear in the settings
view.

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>