Closes #63725

Honor `preview_tabs.enable_preview_from_project_panel` when opening
files with `project_panel::Open` (Space), matching the setting-aware
single-click behavior. Add a regression test covering previews disabled.

Testing:

- `cargo test -p project_panel --lib`
- `cargo build -p zed`
- Manually verified with the setting disabled that Space opens two
permanent tabs
- Manually verified with the setting enabled that Space replaces the
previous preview tab
- `./script/clippy -p project_panel --tests` is blocked by an unrelated
existing `clippy::drain_collect` diagnostic in
`crates/rpc/src/proto_client.rs`

Manual verification:

Preview disabled — Space opens both files as permanent tabs:

<img width="1920" height="1032" alt="Preview disabled: Space opens both
files as permanent tabs"
src="https://github.com/user-attachments/assets/4b772aad-031e-401d-ac85-5a7f82a74bc1"
/>

Preview enabled — Space replaces the previous preview tab:

<img width="1920" height="1032" alt="Preview enabled: Space replaces the
previous preview tab"
src="https://github.com/user-attachments/assets/cfef36af-0cd5-4522-99b5-99d3bb2c3f9d"
/>

Release Notes:

- Fixed the project panel preview-tab setting being ignored when opening
files with the keyboard.

---------

Co-authored-by: Chara <249489759+cmdr-chara@users.noreply.github.com>
Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>