[M3] Add SendStreamingMessage and SubscribeToTask over SSE

[rdobrik]

Description

Implement asynchronous streaming support over SSE for task updates and method streaming responses.

Tasks

• Add SendStreamingMessage support.
• Add SubscribeToTask support.
• Add SSE event publisher and subscriber lifecycle handling.
• Implement terminal-state completion behavior.
• Add timeout/backpressure protections.

Acceptance Criteria

• SSE clients receive ordered updates for target tasks.
• Subscriptions close cleanly on terminal states.
• Integration tests validate connect/stream/complete.

Dependencies

• Issue 7