Execution API set_xcom endpoint returns non-RFC 9457 error for invalid value

### Under which category would you file this issue? Airflow Core ### Apache Airflow version main ### What happened and how to reproduce it? **Issue Description** In the Task Execution API, when a worker attempts to set an XCom via the `set_xcom` endpoint and the underlying `XComModel.set()` raises a `ValueError` (for example, if the DAG run or task instance cannot be resolved), the API currently rejects the request and raises an `HTTPException` with a `404 Not Found` status. However, the `detail` returned is just the raw exception string: `raise HTTPException(status.HTTP_404_NOT_FOUND, str(e))` This violates the Airflow RFC 9457 error response requirements for the Execution API, which mandate that domain-layer exceptions should be caught and returned as structured detail schemas (e.g., `{"reason": "...", "message": "..."}`). **Steps to reproduce:** 1. Send a POST request to `/execution/xcoms/{dag_id}/{run_id}/{task_id}/{key}` with parameters that cause `XComModel.set()` to raise a `ValueError`. 2. Observe the 404 response payload format and see that it is just a plain string instead of a valid RFC 9457 dictionary. ### What you think should happen instead? The exception handler should return an RFC 9457 compliant detail dictionary, for example: ```python raise HTTPException( status_code=status.HTTP_404_NOT_FOUND, detail={"reason": "not_found", "message": str(e)}, ) ``` ### Operating System Not Applicable ### Deployment None ### Apache Airflow Provider(s) _No response_ ### Versions of Apache Airflow Providers _No response_ ### Official Helm Chart version Not Applicable ### Kubernetes Version Not Applicable ### Helm Chart configuration Not Applicable ### Docker Image customizations Not Applicable ### Anything else? _No response_ ### Are you willing to submit PR? - [x] Yes I am willing to submit a PR! ### Code of Conduct - [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)