Delete a span by span_identifier

Delete a single span by identifier.

    **Important**: This operation deletes ONLY the specified span itself and does NOT
    delete its descendants/children. All child spans will remain in the trace and
    become orphaned (their parent_id will point to a non-existent span).

    Behavior:
    - Deletes only the target span (preserves all descendant spans)
    - If this was the last span in the trace, the trace record is also deleted
    - If the deleted span had a parent, its cumulative metrics (error count, token counts)
      are subtracted from all ancestor spans in the chain

    **Note**: This operation is irreversible and may create orphaned spans.