[M2] Implement core method processors: SendMessage, GetTask, ListTasks, CancelTask

[rdobrik]

Description

Add processors for baseline A2A task operations and wire them through method dispatch routes.

Tasks

• Implement processor for SendMessage.
• Implement processor for GetTask.
• Implement processor for ListTasks.
• Implement processor for CancelTask.
• Add request validation per method.

Acceptance Criteria

• Methods are callable via JSON-RPC route.
• Invalid params return -32602.
• Unknown method returns -32601.
• Method contract tests pass.

Dependencies

• Issues 2, 3, 5