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/app-server-protocol/src/lib.rs

@@ -1,7 +1,7 @@
 mod experimental_api;
 mod export;
-mod jsonrpc_lite;
 mod protocol;
+pub mod rpc;
 mod schema_fixtures;
 
 pub use experimental_api::*;
@@ -12,7 +12,6 @@ pub use export::generate_json_with_experimental;
 pub use export::generate_ts;
 pub use export::generate_ts_with_options;
 pub use export::generate_types;
-pub use jsonrpc_lite::*;
 pub use protocol::common::*;
 pub use protocol::event_mapping::*;
 pub use protocol::item_builders::*;
@@ -40,6 +39,7 @@ pub use protocol::v1::SandboxSettings;
 pub use protocol::v1::Tools;
 pub use protocol::v1::UserSavedConfig;
 pub use protocol::v2::*;
+pub use rpc::*;
 pub use schema_fixtures::SchemaFixtureOptions;
 #[doc(hidden)]
 pub use schema_fixtures::generate_typescript_schema_fixture_subtree_for_tests;