Define the code-mode host gRPC protocol (#37510)

## What changed

- Add the `codex.code_mode.v1` protobuf API for managing code-mode sessions, executions, waits, tool callbacks, notifications, and content results.
- Generate and export the Rust client/server bindings with `tonic` under Cargo.
- Add Bazel protobuf targets and a `prost` toolchain that uses the workspace's Rust runtime versions.

GitOrigin-RevId: 51d9d044e01dab2f4d25ab79aa7a52585485eee3

codex-rs/code-mode-protocol/build.rs

@@ -0,0 +1,17 @@
+use std::path::PathBuf;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    println!("cargo:rustc-check-cfg=cfg(codex_bazel)");
+    println!("cargo:rerun-if-changed=src/grpc");
+
+    let mut config = tonic_prost_build::Config::new();
+    config.protoc_executable(protoc_bin_vendored::protoc_bin_path()?);
+    let proto_files = glob::glob("src/grpc/*.proto")?.collect::<Result<Vec<_>, _>>()?;
+
+    tonic_prost_build::configure()
+        .build_client(/*enable*/ true)
+        .build_server(/*enable*/ true)
+        .compile_with_config(config, &proto_files, &[PathBuf::from("src/grpc")])?;
+
+    Ok(())
+}