protocol: separate app and exec RPC ownership (#29714)

## Why

The app-server and exec-server expose separate JSON-RPC APIs, but
exec-server currently sources its serialized protocol and envelope types
through app-server-oriented code. Giving each API an explicit owner
makes the crate boundary legible without introducing shared generic
envelopes.

## What changed

- Added `codex-exec-server-protocol` to own exec DTOs, process IDs, and
JSON-RPC envelopes.
- Updated exec-server clients, transports, handlers, and tests to use
the new crate.
- Exposed app-server's existing JSON-RPC types through a public `rpc`
module while retaining root re-exports.
- Preserved existing wire shapes, including exec `PathUri` behavior.

## Stack

This is PR 1 of 6. Next: [PR
#29721](https://github.com/openai/codex/pull/29721), which moves auth
mode below the app wire boundary.

## Validation

- Exec-server protocol and server coverage passed in the focused
protocol test runs.
- App-server protocol schema fixtures passed.

codex-rs/exec-server/tests/process.rs

@@ -2,8 +2,6 @@
 
 mod common;
 
-use codex_app_server_protocol::JSONRPCMessage;
-use codex_app_server_protocol::JSONRPCResponse;
 use codex_exec_server::ExecResponse;
 use codex_exec_server::InitializeParams;
 use codex_exec_server::InitializeResponse;
@@ -12,6 +10,8 @@ use codex_exec_server::ReadResponse;
 use codex_exec_server::TerminateResponse;
 use codex_exec_server::WriteResponse;
 use codex_exec_server::WriteStatus;
+use codex_exec_server_protocol::JSONRPCMessage;
+use codex_exec_server_protocol::JSONRPCResponse;
 use codex_utils_path_uri::PathUri;
 use common::exec_server::exec_server;
 use pretty_assertions::assert_eq;