Wire SubscribeToTask into the TUI

[alDuncanson]

SubscribeToTask is implemented in the service layer and exposed as handler task resubscribe, but the TUI never calls it. A TUI user who loses a connection, or who wants to attach to a task started elsewhere, has no way back to a running task.

Where it stands

A2AService.resubscribe(task_id) exists and works. tui/server/tab.py does not reference it.

This is the last of the eleven spec methods missing from the TUI, which now covers SendMessage, SendStreamingMessage, GetTask, and CancelTask after #98.

The work

The turn layer added in #98 is the natural home. A resubscribe is a turn without an outgoing message: same streaming render, same cancel affordance, same settling rules. AgentTurn currently always sends — it would need to accept "attach to this task instead".

• Extend the turn layer to drive a subscription as well as a send
• TUI: reconnect to the saved task when a session resumes and the task is still open
• Consider an explicit action for attaching to a task by id

Acceptance

• Resuming a session with a running task reattaches and streams its remaining updates
• Cancel works on a reattached task
• A task that finished while disconnected settles immediately rather than hanging

Part of the A2A v1.0 parity work.

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_{Context: Handler was audited against the A2A v1.0 specification after the v0.2.0 release. Method coverage at the time: service 7/11, CLI 7/11, TUI 4/11 (after #98), MCP 6/11. The four methods missing everywhere are ListTasks, ListTaskPushNotificationConfigs, DeleteTaskPushNotificationConfig, and GetExtendedAgentCard. A deterministic streaming test agent lives outside the repo at ~/code/handler-streaming-agent — it supports ask (input-required), slow (cancellable), and fail, and its card sets capabilities.streaming = true, which ADK does not do by default.}