[Observability AI Assistant] Input box can be resized off-screen (ela…

…stic elastic#181408) (elastic#197063)

fixes [181408](elastic#181408)

## Summary

This PR addresses the issue where the Observability AI Assistant input
box can be resized outside the boundaries of the browser window. Allows
the input box to resize dynamically while ensuring it remains usable,
even when the cursor moves outside the screen during resizing.

x-pack/packages/kbn-ai-assistant/src/prompt_editor/prompt_editor_natural_language.tsx

@@ -109,6 +109,15 @@ export function PromptEditorNaturalLanguage({
     }
   }, [handleResizeTextArea, prompt]);
 
+  useEffect(() => {
+    // Attach the event listener to the window to catch mouseup outside the browser window
+    window.addEventListener('mouseup', handleResizeTextArea);
+
+    return () => {
+      window.removeEventListener('mouseup', handleResizeTextArea);
+    };
+  }, [handleResizeTextArea]);
+
   return (
     <EuiInputPopover
       display="flex"