feat: introduce mcp-types crate (#787)

This adds our own `mcp-types` crate to our Cargo workspace. We vendor in
the
[`2025-03-26/schema.json`](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/05f204513641c05bd78d056791af99c6c84520fa/schema/2025-03-26/schema.json)
from the MCP repo and introduce a `generate_mcp_types.py` script to
codegen the `lib.rs` from the JSON schema.

Test coverage is currently light, but I plan to refine things as we
start making use of this crate.

And yes, I am aware that
https://github.com/modelcontextprotocol/rust-sdk exists, though the
published https://crates.io/crates/rmcp appears to be a competing
effort. While things are up in the air, it seems better for us to
control our own version of this code.

Incidentally, Codex did a lot of the work for this PR. I told it to
never edit `lib.rs` directly and instead to update
`generate_mcp_types.py` and then re-run it to update `lib.rs`. It
followed these instructions and once things were working end-to-end, I
iteratively asked for changes to the tests until the API looked
reasonable (and the code worked). Codex was responsible for figuring out
what to do to `generate_mcp_types.py` to achieve the requested test/API
changes.

Author: bolinfest

codex-rs/Cargo.lock

@@ -7,6 +7,7 @@ members = [
     "core",
     "exec",
     "execpolicy",
+    "mcp-types",
     "tui",
 ]
 

codex-rs/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "mcp-types"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"

codex-rs/mcp-types/Cargo.toml

@@ -0,0 +1,8 @@
+# mcp-types
+
+Types for Model Context Protocol. Inspired by https://crates.io/crates/lsp-types.
+
+As documented on https://modelcontextprotocol.io/specification/2025-03-26/basic:
+
+- TypeScript schema is the source of truth: https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/2025-03-26/schema.ts
+- JSON schema is amenable to automated tooling: https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/2025-03-26/schema.json