Prepare an isolated browser for a session [write]

Prepare before sending the task's first turn. Requires an active session, positive credit balance and remaining session budget. An identical repeated request reuses the browser; a different configuration or an ended browser returns 409. The browser has a 15-minute lifetime, fixed viewport and bounded actions. Structured Google Flights verification checks visible route, dates, passengers, cabin and results; tasks without verification remain unverified. Browser usage is billed separately from model and runtime usage.