Fixes #8048

## Summary

In vim mode, pressing Escape to dismiss the buffer search now correctly
restores the cursor to its original position, rather than leaving it at
the first match.

## Problem

When using vim's `/` command to search:
1. User positions cursor at line X
2. User presses `/` to open search, types a query
3. Matches are highlighted, cursor may visually jump to first match
4. User presses Escape to dismiss without navigating
5. **Bug:** Cursor ends up at first match instead of line X

This breaks vim parity where Escape should cancel the search and restore
cursor position.

## Solution

The fix leverages the `focused()` callback in `vim.rs`, which is called
when the editor regains focus after the search bar is dismissed.

**Key insight:** When search starts via `/`, the cursor position is
saved in `SearchState.prior_selections`. When search is *submitted* with
Enter, `search_submit()` drains these selections. But when search is
*dismissed* with Escape, they remain.

So in `focused()`, if:
- `prior_selections` is non-empty, AND
- The search bar's `is_dismissed()` returns true

...then we know the user dismissed the search (Escape) rather than
submitted it (Enter), and we restore the cursor.

## Why not handle `buffer_search::Dismiss` directly?

The initial approach tried to register a vim handler for the `Dismiss`
action. This didn't work because when Escape is pressed, the search bar
(which has focus) handles the `Cancel` action internally and calls its
`dismiss()` method directly—it doesn't dispatch `Dismiss` through the
action system. The vim handler registered on the editor was never
invoked.

## Test Plan

- Added `test_search_dismiss_restores_cursor` — verifies cursor
restoration when search is dismissed
- Added `test_search_dismiss_restores_cursor_no_matches` — verifies
behavior when query has no matches
- All 455 vim tests pass
- Manual testing confirms fix works with both `/` and `cmd-f`

## Release Notes

- Fixed vim mode: cursor now returns to original position when
dismissing buffer search with Escape (#8048)

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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>