## What changed
- Add `McpEventStream::open` to construct event stream requests and keep the
underlying `RmcpClient` alive until the subscription completes or is canceled.
- Stop retaining the broader `McpServerConnection` for the lifetime of an event
stream, and remove the connection-returning lookup helper.
- Use MCP event server terminology in event discovery and subscription errors.
GitOrigin-RevId: 7d6c7b8ed9c5447d2d8cda4a077da845fb026173