Concurrent/retried Execution API XCom writes fail with 409 due to duplicate primary key

### Under which category would you file this issue? Task SDK ### Apache Airflow version 3.2.2 ### What happened and how to reproduce it? An Airflow task runner attempted to store an XCom through the Airflow 3 Execution API: POST /execution/xcoms/{dag_id}/{run_id}/{task_id}/{key} The API server attempted the following statement: INSERT INTO xcom (dag_run_id, task_id, map_index, `key`, dag_id, run_id, value, timestamp) VALUES (%s, %s, %s, %s, %s, %s, %s, %s) The insert failed with an IntegrityError and the Execution API returned: HTTP 409 Conflict Relevant access log: [2026-07-16T07:14:19.285110Z] request finished method=POST path=/execution/xcoms/{Run_ID}/runOnce__2026-07-16T07:01:26.419019+00:00/{Task_SENSOR}/{Task_SENSOR} status_code=409 Three XCom INSERT failures occurred at approximately the same time. The Airflow API exception handler classified the MySQL IntegrityError as: "Unique constraint violation" The XCom primary key is: (dag_run_id, task_id, map_index, key) In Airflow 3.2.2, `XComModel.set()` first deletes an existing XCom and then inserts the replacement: 1. DELETE matching XCom 2. INSERT new XCom With overlapping requests, both transactions can perform the DELETE before either INSERT commits. One INSERT succeeds and another fails with a duplicate primary-key error. The approximately 81-second request duration may indicate that one INSERT waited for another database transaction before the duplicate conflict was returned. ### What you think should happen instead? Repeated or concurrent writes of the same XCom identity should be handled idempotently. Possible expected behavior: - Use a database-appropriate atomic upsert. - Serialize writes for the same XCom identity. - If a repeated request contains the same value, treat it as already completed. - Retry the write safely after an IntegrityError caused by the delete-then-insert race. A transient/repeated Execution API request should not fail task execution merely because the same XCom was already writte…