The developer extension should respect cancellation messages, killing any running processes

[jamadeo]

If the user hits "stop" during in-flight tool calls, we'll send a cancellation notification through the MCP client:

goose/crates/mcp-client/src/client.rs

Line 206 in b000911

send_cancel_message(&peer, request_id, Some("operation cancelled".to_owned())).await?;

On the other side though, our developer MCP extension does not handle this. We should have it do so. When the developer extension gets a cancellation message, it should kill any running processes that it owns.

The "right" way to make this happen is probably to move the developer extension over to an rmcp implementation.