Bug/XCom sequence slice endpoint returns wrong rows for crossed-bound slices (negative SQL LIMIT)

### Under which category would you file this issue? Airflow Core ### Apache Airflow version main (3.4.0.dev0), reproduced at commit a3f51fcd9c ### What happened and how to reproduce it? ## Issue Description The mapped XCom sequence-slice endpoint in the Execution API returns incorrect results for **crossed-bound slices**—cases where the effective `stop` index comes before the effective `start` index. According to Python slicing semantics, these slices should always evaluate to an empty list. **Endpoint** ```text GET /execution/xcoms/{dag_id}/{run_id}/{task_id}/{key}/slice?start=&stop=&step= ``` **Handler** ```text airflow-core/src/airflow/api_fastapi/execution_api/routes/xcoms.py get_mapped_xcom_by_slice() ``` The handler is intended to reproduce Python list slicing over stored XCom rows, but it translates the slice into SQLAlchemy's `Query.slice(a, b)`, which generates: ```sql OFFSET a LIMIT (b - a) ``` Unlike Python slicing, SQLAlchemy does **not** clamp negative limits. When `b < a`, the generated `LIMIT` becomes negative, resulting in backend-dependent behavior. ### Backend behavior - **SQLite** - A negative `LIMIT` is treated as **"no limit"**. - The query silently returns rows from `OFFSET` onward. - **Result:** incorrect data is returned. - **PostgreSQL / MySQL** - Negative `LIMIT` values are rejected. - **Result:** the request fails with a server error. The endpoint should instead return an empty result (`[]`) for all crossed-bound slices, matching Python semantics. --- ## Examples Assume six stored XCom values: ```text ["v0", "v1", "v2", "v3", "v4", "v5"] ``` ### Case 1 Request: ```text start=2&stop=-3&step=-1 ``` Python: ```python list(range(6))[2:-3:-1] # [] ``` Current implementation: ```text stop -> 3 slice(4, 3) OFFSET 4 LIMIT -1 ``` SQLite returns: ```text ["v5", "v4"] ``` Expected: ```text [] ``` --- ### Case 2 Request: ```text start=4&stop=2 ``` Python: ```python list(range(6))[4:2] # [] ``` Current implementation: ```text slice(4, 2) OFFS…