Closes #4864

Adds a `file_finder::OpenWithoutDismiss` action that opens the selected
file without closing the finder, so multiple files can be opened in one
session. Pressing right arrow when the cursor is at the end of the
search query triggers it — matching VS Code and Sublime Text behaviour.
A "Keep Open" button in the footer provides mouse access and serves as a
discoverability hint.

Each opened file becomes the active tab without stealing keyboard focus
from the finder, preserving the search state between opens. Refactors
the existing `confirm` implementation into a shared `open_selected_file`
helper parameterised by `dismiss_after_open` to eliminate duplicated
file-opening logic.

## What's different from PR #38914

- Uses `right` arrow at `end_of_input` context instead of
`cmd-shift-enter` (better ergonomics, matches VS Code/Sublime — the
`end_of_input` context that was added after that PR makes this possible)
- Each opened file is activated as the current tab (`activate: true`,
`focus_item: false`), giving visual feedback on every open without
stealing focus from the modal
- Cleaner refactor: single `open_selected_file` helper instead of
duplicated logic

Release Notes:

- Added ability to open files from the file finder without dismissing
it, using right arrow at end of input or the "Keep Open" footer button

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Co-authored-by: Christopher Biscardi <chris@christopherbiscardi.com>