Users can now Cmd+click multiple files in the file picker and press
Enter to open them all at once. Tab can also be used to select items
without clicking, advancing the cursor automatically.

# Objective


Currently, when opening the file picker (Cmd+P), users can only open one
file at a time. This adds multi-select support so users can select
multiple files and open them all at once.

Fixes #59818

## Solution

Added a `selected_indices` set to `Picker` to track which items the user
has toggled into a multi-selection. Regular clicks still open a file
immediately (unchanged behavior).
Cmd+clicking adds a file to the selection without opening it, and
pressing Enter opens all selected files at once. A `confirm_multi`
method was added to the delegate trait so individual delegates (like the
file finder) can handle opening multiple files with a safe fallback for
delegates that don't support it. Multi-selection is cleared when the
picker
is dismissed or the search query changes. Selected items show a
background highlight and a focused left border so the user always knows
what's selected.



## How to use it

1. Open the file or test picker.
2. Search and either Cmd+click or tab it to add it to the selection (the
file will not open yet).
3. Repeat for any other files you want to open.
4. To deselect a file, Cmd+click it again.
5. Alternatively, hover over an item and press Tab to select it with the
keyboard, the cursor automatically moves to the next item so you can
keep selecting quickly.
6. Once you have all the files you want, press Enter to open them all at
once.


## Testing

Tested manually on macOS.

- **Single select:** Clicking on a file opens it directly, which is the
existing behavior and remains unchanged.
- **Multi-select via Cmd+Click:** Holding Cmd and clicking multiple
items adds them to the selection. Cmd+clicking an already-selected item
deselects it. Pressing Enter opens all selected files at once.
- **Multi-select via Tab:** Hovering over an item and pressing Tab
selects it without clicking or opening it, allowing keyboard-driven
multi-selection.

## Showcase

**Single select:** 
<img width="1512" height="982" alt="Screenshot 2026-06-25 at 6 45 33 PM"
src="https://github.com/user-attachments/assets/8f9d4408-0688-4619-807a-affd6dc66f21"
/>








Opened single file

<img width="1512" height="982" alt="Screenshot 2026-06-25 at 6 45 38 PM"
src="https://github.com/user-attachments/assets/34ffacfd-b8ed-4898-beaa-bd5e47ebf682"
/>












- **Multi-select:**
- Selecting multiple files
<img width="1512" height="982" alt="Screenshot 2026-06-25 at 6 49 22 PM"
src="https://github.com/user-attachments/assets/a7562fe8-9bd2-40ba-8bb4-12909ccefed7"
/>









Opened multiple files

<img width="1512" height="982" alt="Screenshot 2026-06-25 at 6 49 42 PM"
src="https://github.com/user-attachments/assets/c0c9e24c-86b7-45bf-898d-05068d5d7e3a"
/>





## Release Notes:
- Added multi-select to the file and text picker: Cmd+click or tab to
select multiple files, then open them all at once.

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Yara <git@yara.blue>