Offer asyncTasks to clients that are not JetBrains AIR #645

Could async tasks be negotiated independently of AIR rendering? In v2.2.2, receiving async_task_spawned and async_task_state_update requires clientCapabilities._meta.jetbrains.air = {version: 1, capabilities: ['asyncTasks']}; the extension also provides _session/async_task/stop. See capability validation and async-task contract.

The presence of the AIR object also selects AIR rendering independently of the capability list (client detection). That changes tool fields and output placement; for example, read/search/list command output moves to rawOutput, and guardian-review input and results use a different shape (tool contracts). An ACP v1 client that builds permission previews from rawInput and relies on the existing tool contract cannot opt into async tasks alone.

Would you consider a standalone async-task capability, or a capability under _meta that enables task updates and stop requests without selecting AIR rendering? Related: #492 asks for a per-task stop. Thank you.