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.