These changes are a follow-up to #58244. That PR changed how Zed sends
`textDocument/definition` requests so that, once the language server has
answered for a given position, Zed doesn't re-query that same position
as the mouse jiggles within the same character, avoiding spamming the
server on every tiny mouse movement.

This PR adds two more cases on top of that:

**1. Preserve the deduplication state when the server returns an empty
response**

* Previously, an empty response called `Editor::hide_hovered_link`,
which sets `Editor::hovered_link_state` to `None`, discarding the
recorded `HoveredLinkState::last_trigger_point`.
* We now clear `HoveredLinkState::links` instead, resetting the links
while keeping `last_trigger_point`, so we don't re-query the same
position when the response was empty.

**2. Fall back to the surrounding word as the symbol range for no-origin
servers**

* The `editor::hover_links::show_link_definition` function already used
`surrounding_word` to pick the highlight range when the server returned
no symbol range.
* For servers that return a link (or links) but no
`originSelectionRange`, we now also treat that `surrounding_word` range
as the symbol range, so moving across characters of the same symbol will
not re-query the language server.
* There's caveat to this approach, for a language where something like
`run_function` is not a single symbol/node, if the server returns a
response for one character, we won't re-query for the others in
`run_function`, even if the server would have returned something
different for `_` or `function`. I believe this should be rare, can't
think of a language where this would be an issue, but I'm happy to
revisit or drop this part if we decide it's not worth it.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Improve go to definition in order to avoid repeatedly querying the
language server while moving the mouse over the same symbol or an
unresolved position