fix sample agent request command and payload

[vikions]

Summary

• update make sample to use the current /api/v2/agent/run endpoint
• make sample-payload.json valid JSON
• add the required captchaToken field to the sample payload
• allow the sample command to send either a Firebase bearer token or local skip-auth headers from env vars

Why

The local sample request had drifted from the current FastAPI API shape:

• the Makefile sample command still pointed to the old route
• the bundled sample payload was invalid JSON
• the payload was missing fields now expected by the current request flow

Validation

• python -m json.tool sample-payload.json

Scope

• does not change Python version usage in Makefile
• does not touch broader stale test targets in this PR