fix: use macros to ensure request/response symmetry (#4529)

Manually curating `protocol-ts/src/lib.rs` was error-prone, as expected.
I finally asked Codex to write some Rust macros so we can ensure that:

- For every variant of `ClientRequest` and `ServerRequest`, there is an
associated `params` and `response` type.
- All response types are included automatically in the output of `codex
generate-ts`.

codex-rs/Cargo.lock

@@ -1103,6 +1103,7 @@ dependencies = [
  "icu_locale_core",
  "mcp-types",
  "mime_guess",
+ "paste",
  "pretty_assertions",
  "serde",
  "serde_json",