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/Cargo.toml

@@ -178,7 +178,7 @@ ratatui-macros = "0.6.0"
 regex = "1.12.2"
 regex-lite = "0.1.7"
 reqwest = "0.12"
-rmcp = { version = "0.10.0", default-features = false }
+rmcp = { version = "0.12.0", default-features = false }
 schemars = "0.8.22"
 seccompiler = "0.5.0"
 sentry = "0.46.0"