editor: Add diffs_expanded to key context when diff hunks are expanded (#40617)

including a new identifier on the Editor key context will allow for some
more flexibility when creating keybindings.

for example i would like to be able to set the following:
```json
{
  "context": "Editor",
  "bindings": {
    "pageup": ["editor::MovePageUp", { "center_cursor": true }],
    "pagedown": ["editor::MovePageDown", { "center_cursor": true }],
  }
},
{
  "context": "Editor && diffs_expanded",
  "bindings": {
    "pageup": "editor::GoToPrevHunk",
    "pagedown": "editor::GoToHunk",
  }
},
```

<img width="1392" height="1167" alt="Screenshot 2025-10-18 at 23 51 46"
src="https://github.com/user-attachments/assets/cf4e262e-97e7-4dd9-bbda-cd272770f1ac"
/>


very open to suggestions for the name. that's the best i could come up
with.

the action *IS* called `editor::ExpandAllDiffHunks` so this seems
fitting.

the identifier is included if *any* diff hunk is visible, even if some
of them have been closed using `editor::ToggleSelectedDiffHunk`


Release Notes:

- The Editor key context now includes 'diffs_expanded' when diff changes
are visible

crates/editor/src/editor.rs

@@ -2631,6 +2631,10 @@ impl Editor {
             key_context.add("end_of_input");
         }
 
+        if self.has_any_expanded_diff_hunks(cx) {
+            key_context.add("diffs_expanded");
+        }
+
         key_context
     }
 
@@ -19336,6 +19340,16 @@ impl Editor {
         })
     }
 
+    fn has_any_expanded_diff_hunks(&self, cx: &App) -> bool {
+        if self.buffer.read(cx).all_diff_hunks_expanded() {
+            return true;
+        }
+        let ranges = vec![Anchor::min()..Anchor::max()];
+        self.buffer
+            .read(cx)
+            .has_expanded_diff_hunks_in_ranges(&ranges, cx)
+    }
+
     fn toggle_diff_hunks_in_ranges(
         &mut self,
         ranges: Vec<Range<Anchor>>,