Pick up the published ACP Rust SDK 2.2.0 release, including the schema
1.9.1 compatibility fix for agents returning `null` for
empty/defaultable payloads.

Upstream changes:
- https://github.com/agentclientprotocol/agent-client-protocol/pull/2178
- https://github.com/agentclientprotocol/rust-sdk/pull/367

Release Notes:

- acp: Fixed errors when ACP agents return `null` for empty response
payloads.