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/rmcp-client/src/rmcp_client.rs

@@ -28,7 +28,7 @@ use rmcp::model::CallToolRequestParam;
 use rmcp::model::ClientNotification;
 use rmcp::model::CreateElicitationRequestParam;
 use rmcp::model::CreateElicitationResult;
-use rmcp::model::CustomClientNotification;
+use rmcp::model::CustomNotification;
 use rmcp::model::Extensions;
 use rmcp::model::InitializeRequestParam;
 use rmcp::model::PaginatedRequestParam;
@@ -372,13 +372,11 @@ impl RmcpClient {
         let service: Arc<RunningService<RoleClient, LoggingClientHandler>> = self.service().await?;
         service.service();
         service
-            .send_notification(ClientNotification::CustomClientNotification(
-                CustomClientNotification {
-                    method: method.to_string(),
-                    params,
-                    extensions: Extensions::new(),
-                },
-            ))
+            .send_notification(ClientNotification::CustomNotification(CustomNotification {
+                method: method.to_string(),
+                params,
+                extensions: Extensions::new(),
+            }))
             .await?;
         Ok(())
     }