Handle on click for split button

the split button wasn't triggering the AI request (but the single button works). That's due to the changes I made after this suggestion !137396 (comment 1660361406).

This is what the fix looks like:

<split-button
  v-if="actionButtons.length > 1"
  :buttons="actionButtons"
  :disabled="isProcessingAction"
  @createMergeRequest="createMergeRequest"
  @downloadPatch="downloadPatch"
- @createMergeRequestAi="startSubscription"
+ @startSubscription="startSubscription"
/>