Download a file an A2A task wrote

A2A 1.0 JSON-RPC, for a subject, under the Nylorun profile v2 (Host feature `a2a-profile-v2`). `SendMessage` blocks up to the Runtime limit `a2aWaitMs` (5 minutes by default; then the task is answered still `WORKING`) unless `returnImmediately`; `GetTask` (with `historyLength`), `CancelTask` and `ListTasks`; and the four push notification config methods, plus `configuration.taskPushNotificationConfig` in `SendMessage`: each update is POSTed as one `StreamResponse`, signed with Standard Webhooks (`webhook-id`, `webhook-timestamp`, `webhook-signature` with the Runtime's `a2a.push.secret`) and `X-A2A-Notification-Token`, at least once, retried for up to 24 hours, to hosts the Runtime allows (`a2a.push.allowHosts` for private ones). A message to a busy context is queued and answered with the task that will take it (`TASK_STATE_SUBMITTED`); `message.metadata.delivery: "reject"` asks for `UnsupportedOperationError` (reason `CONTEXT_BUSY`) instead. A message is text, one data part, or text with `raw` files (stored as artifacts of the context's session) and `url` files (fetched by the Runtime), each at most the Runtime limit `inlineFileBytes` (20 MiB by default). An approval, question or connection the task waits for (`AUTH_REQUIRED` or `INPUT_REQUIRED`) is shown under `interactions/v1` and answered on the same task with `{ interactionId, approved }` or `{ interactionId, value }` under that URI in the message's `metadata`; text never approves. Artifacts carry the turn's output and the files it wrote, as `url` parts at `/v1/a2a/files/…` with `files/v1`; the task's metadata carries `progress/v1`, `telemetry/v1` and `effects/v1`. Extension data is answered only for the extensions activated with `A2A-Extensions`. A context is one session per subject, agent and `contextId`; a task is one turn. JSON-RPC errors are answered with 200; limits, scopes and unavailable streams stay HTTP errors. Not served: streaming, `GetExtendedAgentCard`.