feat: add typed TaskStarted/TaskProgress/TaskNotification message subclasses

[qing-ant]

Adds typed dataclass subclasses of SystemMessage for task-related messages, matching the TypeScript SDK's SDKTaskStartedMessage/SDKTaskProgressMessage/SDKTaskNotificationMessage.

Approach

The three new classes subclass SystemMessage so existing code is unaffected:

• isinstance(msg, SystemMessage) → True for all typed task messages
• msg.subtype and msg.data remain populated
• case SystemMessage(): pattern matching still works
• Unknown system subtypes still yield generic SystemMessage (not accidentally subclassed)

The parser dispatches on subtype to emit the specific subclass with typed fields; falls back to generic SystemMessage for anything else.

New types

• TaskStartedMessage(SystemMessage) — task_id, description, uuid, session_id, tool_use_id?, task_type?
• TaskProgressMessage(SystemMessage) — task_id, description, usage: TaskUsage, uuid, session_id, tool_use_id?, last_tool_name?
• TaskNotificationMessage(SystemMessage) — task_id, status: 'completed'|'failed'|'stopped', output_file, summary, uuid, session_id, tool_use_id?, usage?
• TaskUsage TypedDict — {total_tokens, tool_uses, duration_ms}

• Add typed TaskStartedMessage, TaskProgressMessage, TaskNotificationMessage subclasses of SystemMessage

Test plan

• Unit: parser dispatch for all three subtypes, optional-field handling, isinstance(SystemMessage) and pattern-match backward-compat, generic fallback for unknown subtypes
• E2E: real CLI subagent spawned — observed 1x TaskStartedMessage, 4x TaskProgressMessage, 1x TaskNotificationMessage, all fields correctly populated, init subtype correctly remained plain SystemMessage