callback: Get handlers with json callback plugin (#35842)

Implement the `v2_playbook_on_handler_task_start()' method in the json
callback plugin to correctly include handlers in the results.

This fixes a bug where the last task in the results returned by
json callback would be wrong if an unconditional handler was triggered,
since the result of that handler would overwrite the result of the last
task.

Signed-off-by: Alex Tsitsimpis <alextsi@arrikto.com>

lib/ansible/plugins/callback/json.py

@@ -55,6 +55,9 @@ class CallbackModule(CallbackBase):
     def v2_playbook_on_task_start(self, task, is_conditional):
         self.results[-1]['tasks'].append(self._new_task(task))
 
+    def v2_playbook_on_handler_task_start(self, task):
+        self.results[-1]['tasks'].append(self._new_task(task))
+
     def v2_runner_on_ok(self, result, **kwargs):
         host = result._host
         self.results[-1]['tasks'][-1]['hosts'][host.name] = result._result