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/http_request.rs

@@ -6,16 +6,16 @@ use std::collections::BTreeMap;
 use std::io::ErrorKind;
 use std::time::Duration;
 
-use codex_app_server_protocol::JSONRPCError;
-use codex_app_server_protocol::JSONRPCMessage;
-use codex_app_server_protocol::JSONRPCNotification;
-use codex_app_server_protocol::JSONRPCResponse;
-use codex_app_server_protocol::RequestId;
 use codex_exec_server::HttpHeader;
 use codex_exec_server::HttpRequestBodyDeltaNotification;
 use codex_exec_server::HttpRequestParams;
 use codex_exec_server::HttpRequestResponse;
 use codex_exec_server::InitializeParams;
+use codex_exec_server_protocol::JSONRPCError;
+use codex_exec_server_protocol::JSONRPCMessage;
+use codex_exec_server_protocol::JSONRPCNotification;
+use codex_exec_server_protocol::JSONRPCResponse;
+use codex_exec_server_protocol::RequestId;
 use common::exec_server::ExecServerHarness;
 use common::exec_server::exec_server;
 use pretty_assertions::assert_eq;
@@ -392,7 +392,7 @@ where
 async fn wait_for_error_response(
     server: &mut ExecServerHarness,
     request_id: RequestId,
-) -> anyhow::Result<codex_app_server_protocol::JSONRPCErrorError> {
+) -> anyhow::Result<codex_exec_server_protocol::JSONRPCErrorError> {
     let response = server
         .wait_for_event(|event| {
             matches!(