Create session
POST/rest/v1/:organisationId/:environmentId/agents/:agentId/sessions
Start a session for the agent and return it under data.session with status in_progress. Every body field is optional, but a body is required, so send {} to use the defaults. sessionId is generated if omitted, and reusing an existing ID overwrites that session. Returns 201. Requires the agentic-runtime-service:session:create scope.