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

bazel/toolchains/prost/BUILD.bazel

@@ -0,0 +1,35 @@
+load("@rules_rust//extensions/prost:defs.bzl", "rust_prost_toolchain")
+load("@rules_rust//rust:defs.bzl", "rust_library_group")
+
+package(default_visibility = ["//visibility:public"])
+
+rust_library_group(
+    name = "prost_runtime",
+    deps = ["@crates//:prost"],
+)
+
+rust_library_group(
+    name = "tonic_runtime",
+    deps = [
+        ":prost_runtime",
+        "@crates//:tonic",
+        "@crates//:tonic-prost",
+    ],
+)
+
+rust_prost_toolchain(
+    name = "toolchain_impl",
+    compile_well_known_types = False,
+    prost_plugin = "@rules_rs//rs/private/prost:protoc-gen-prost__protoc-gen-prost",
+    prost_runtime = ":prost_runtime",
+    prost_types = "@crates//:prost-types-0.14.3",
+    tonic_plugin = "@rules_rs//rs/private/prost:protoc-gen-tonic__protoc-gen-tonic",
+    tonic_plugin_flag = "--plugin=protoc-gen-tonic=%s",
+    tonic_runtime = ":tonic_runtime",
+)
+
+toolchain(
+    name = "toolchain",
+    toolchain = ":toolchain_impl",
+    toolchain_type = "@rules_rust//extensions/prost:toolchain_type",
+)