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/bin/rmcp_test_server.rs

@@ -81,6 +81,7 @@ impl ServerHandler for TestToolServer {
             Ok(ListToolsResult {
                 tools: (*tools).clone(),
                 next_cursor: None,
+                meta: None,
             })
         }
     }