lsp: only notify if hover state changed (#2577)

Follow-up to #2239, which added clear_hover_state. That method always
calls cx.notify(), but on_mouse_move calls it on every mouse move
outside the input bounds, where hover state is normally already empty.
The input was re-rendering on mouse movement anywhere else in the window
for no reason.

Now it only notifies when a hover definition or popover was actually
cleared. The _hover_task reset stays unconditional so an in-flight hover
request is still cancelled during the 150ms debounce in
handle_hover_popover, before any popover exists.

How to Test

Open the code editor story (cargo run, Input Story > Code Editor).

Hover a symbol until the popover shows, then move off it, and out of the
editor. It should dismiss both times. Cmd-hover a symbol for the
go-to-definition underline and confirm that clears too. Hover a symbol
and leave the editor within 150ms; the popover should not appear
afterwards.

Checklist

- [x] I have read the CONTRIBUTING (../CONTRIBUTING.md) document and
followed the guidelines.
- [x] Reviewed the changes in this PR and confirmed AI generated code
(If any) is accurate.
- [x] Passed cargo run for story tests related to the changes.
- [ ] Tested macOS, Windows and Linux platforms performance (if the
change is platform-specific)