Fix XCom slice endpoint returning wrong rows for crossed-bound slices

[dhruv-techdev]

A crossed-bound sequence slice (stop before start) evaluates to an empty list in Python, but the mapped-XCom slice endpoint translated it into a SQLAlchemy .slice() with a negative SQL LIMIT. On SQLite a negative LIMIT means "no limit", so rows were silently returned from the offset onward; on PostgreSQL/MySQL the negative LIMIT is rejected and the request fails. Return an empty result for crossed bounds so behaviour matches Python slicing on every backend.

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