Fix crash when vim editor is not found in PATH on Windows

Fixes #16492

This patch checks if the editor command exists before calling spawnSync.
Without this check, Node throws ENOENT when vim is not in PATH on Windows.

packages/core/src/utils/editor.ts

@@ -302,6 +302,10 @@ export async function openDiff(
 
   if (isTerminalEditor(editor)) {
     try {
+      if (!commandExists(diffCommand.command)) {
+        throw new Error(`Editor command not found: ${diffCommand.command}`);
+      }
+
       const result = spawnSync(diffCommand.command, diffCommand.args, {
         stdio: 'inherit',
       });