assistant: Fix "New Context" behavior when focused in editor (#17106)

Closes #16676

Release Notes:

- assistant: fix `New Context` opening a new file when focused in the
editor pane.

crates/assistant/src/assistant.rs

@@ -63,6 +63,7 @@ actions!(
         DeployHistory,
         DeployPromptLibrary,
         ConfirmCommand,
+        NewContext,
         ToggleModelSelector,
     ]
 );