editor: Add in_preview boolean context for keybindings (#61777)

# Objective

- Add an `in_preview` context that is available when the current editor
is in preview mode (opened from project panel or go to definition).
- closes https://github.com/zed-industries/zed/discussions/61765

## Solution

- In `editor.rs`, added a check in `key_context()` that detects if the
editor is the active preview item in its pane, and if so, adds
`"in_preview"` to the key context.
- Uses existing `workspace.pane_for_item_id()` and
`pane.is_active_preview_item()` methods — no new APIs introduced.

## Testing

- Added two tests in `editor_tests.rs`:
- `test_in_preview_context_added_when_in_preview_mode` — verifies
`in_preview` is present when editor is set as preview item.
- `test_in_preview_context_not_added_when_not_in_preview_mode` —
verifies `in_preview` is absent for regular (non-preview) tabs.
- Tested on Linux.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

---

Release Notes:

- Added `in_preview` keybinding context for when an editor is in preview
mode.

crates/workspace/src/pane.rs

@@ -4661,6 +4661,10 @@ impl Render for Pane {
 }
 
 impl ItemNavHistory {
+    pub fn is_preview_item(&self) -> bool {
+        self.history.0.lock().preview_item_id == Some(self.item.id())
+    }
+
     pub fn push<D: 'static + Any + Send + Sync>(
         &mut self,
         data: Option<D>,
@@ -4672,19 +4676,18 @@ impl ItemNavHistory {
             .upgrade()
             .is_some_and(|item| item.include_in_nav_history())
         {
-            let is_preview_item = self.history.0.lock().preview_item_id == Some(self.item.id());
+            let is_preview_item = self.is_preview_item();
             self.history
                 .push(data, self.item.clone(), is_preview_item, row, cx);
         }
     }
 
     pub fn navigation_entry(&self, data: Option<Arc<dyn Any + Send + Sync>>) -> NavigationEntry {
-        let is_preview_item = self.history.0.lock().preview_item_id == Some(self.item.id());
         NavigationEntry {
             item: self.item.clone(),
             data,
             timestamp: 0,
-            is_preview: is_preview_item,
+            is_preview: self.is_preview_item(),
             row: None,
         }
     }