The XCom endpoint joins the XCom and DagRun tables. Currently, the output of the join isn't getting stored as a new `query` variable, as seen in the link to the following PR. We should add a new test to check that these tables are getting joined correctly.

https://github.com/apache/airflow/pull/10594/files/1fe1da978763c9919cee0d2c7eccd96438631993#diff-aa0c2f68c6177fb1587a6de6b5cfb28cR59


I will make these changes after #10594 
tests/api_connexion/endpoints/test_xcom_endpoint.py
def test_should_respond_200(self):
        dag_id = 'test-dag-id'
        task_id = 'test-task-id'
        execution_date = '2005-04-02T00:00:00+00:00'
        xcom_key = 'test-xcom-key'
        execution_date_parsed = parse_execution_date(execution_date)
        dag_run_id = DR.generate_run_id(DagRunType.MANUAL, execution_date_parsed)
        self._create_xcom_entry(dag_id, dag_run_id, execution_date_parsed, task_id, xcom_key)
        response = self.client.get(
            f"/api/v1/dags/{dag_id}/dagRuns/{dag_run_id}/taskInstances/{task_id}/xcomEntries/{xcom_key}",
            environ_overrides={'REMOTE_USER': "test"},
        )
        self.assertEqual(200, response.status_code)
        current_data = response.json
        current_data['timestamp'] = 'TIMESTAMP'
        self.assertEqual(
            current_data,
            {
                'dag_id': dag_id,
                'execution_date': execution_date,
                'key': xcom_key,
                'task_id': task_id,
                'timestamp': 'TIMESTAMP',
            },
        )
    def test_should_raises_401_unauthenticated(self):