A2A Client Manager in RemoteAgentInvocation

*   Create `packages/core/src/agents/remote-agent-invocation.ts`.
    *   Implements `ToolInvocation`.
    *   `execute()`: Uses `A2AClientManager` (passed from wrapper/registry) to get the client, checks `streaming` capability, calls `sendMessage`/`sendMessageStream`, and returns `ToolResult`. 
 
