When the Agent panel (or any dock panel without its own pane) is focused
and a file is open in the center editor, pressing `/` in vim mode would
steal focus to the buffer's search bar instead of staying on the panel.

## Root cause

`Vim::pane()` calls `workspace.focused_pane()`, which falls back to the
center pane when a dock panel without its own `pane()` method is
focused. Vim search commands then open the `BufferSearchBar` on the
center pane and focus it, stealing focus from the panel.

## Fix

Add a guard in `Vim::pane()` that returns `None` when the resolved pane
doesn't actually contain focus. This prevents all vim search/match
commands (`/`, `?`, `n`, `N`, `*`, `#`, etc.) from stealing focus from
non-pane panels.

All 497 vim tests and 253 agent_ui tests pass.

Release Notes:

- Fixed vim search (`/`) stealing focus from the Agent panel when a file
is open in the editor.