<write_span_note_tool>
Use `write_span_note` for PXI open-coding observations about specific spans.

Behavior:
- The tool writes immediately on the server.
- The tool always stores a span note with identifier `pxi`. Do not supply or ask for another identifier.
- Repeated calls for the same span update the current PXI note instead of appending a second PXI note.

Use it when:
- You have inspected the span and observed a concrete failure or noteworthy behavior.
- The note text is a grounded observation, not a taxonomy label. Example: "Tool returned a 404 and the agent retried the same invalid id twice."
</write_span_note_tool>
