chore: upgrade rmcp crate from 0.10.0 to 0.12.0 (#8288)

Version `0.12.0` includes
https://github.com/modelcontextprotocol/rust-sdk/pull/590, which I will
use in https://github.com/openai/codex/pull/8142.

Changes:

- `rmcp::model::CustomClientNotification` was renamed to
`rmcp::model::CustomNotification`
- a bunch of types have a `meta` field now, but it is `Option`, so I
added `meta: None` to a bunch of things

codex-rs/exec-server/src/posix/mcp.rs

@@ -183,10 +183,10 @@ impl ServerHandler for ExecTool {
 
     async fn on_custom_notification(
         &self,
-        notification: rmcp::model::CustomClientNotification,
+        notification: rmcp::model::CustomNotification,
         _context: rmcp::service::NotificationContext<rmcp::RoleServer>,
     ) {
-        let rmcp::model::CustomClientNotification { method, params, .. } = notification;
+        let rmcp::model::CustomNotification { method, params, .. } = notification;
         if method == MCP_SANDBOX_STATE_NOTIFICATION
             && let Some(params) = params
         {