Add exec-server stub server and protocol docs (#15089)

Stacked PR 1/3.

This is the initialize-only exec-server stub slice: binary/client
scaffolding and protocol docs, without exec/filesystem implementation.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/exec-server/src/server/handler.rs

@@ -0,0 +1,40 @@
+use std::sync::atomic::AtomicBool;
+use std::sync::atomic::Ordering;
+
+use codex_app_server_protocol::JSONRPCErrorError;
+
+use crate::protocol::InitializeResponse;
+use crate::server::jsonrpc::invalid_request;
+
+pub(crate) struct ExecServerHandler {
+    initialize_requested: AtomicBool,
+    initialized: AtomicBool,
+}
+
+impl ExecServerHandler {
+    pub(crate) fn new() -> Self {
+        Self {
+            initialize_requested: AtomicBool::new(false),
+            initialized: AtomicBool::new(false),
+        }
+    }
+
+    pub(crate) async fn shutdown(&self) {}
+
+    pub(crate) fn initialize(&self) -> Result<InitializeResponse, JSONRPCErrorError> {
+        if self.initialize_requested.swap(true, Ordering::SeqCst) {
+            return Err(invalid_request(
+                "initialize may only be sent once per connection".to_string(),
+            ));
+        }
+        Ok(InitializeResponse {})
+    }
+
+    pub(crate) fn initialized(&self) -> Result<(), String> {
+        if !self.initialize_requested.load(Ordering::SeqCst) {
+            return Err("received `initialized` notification before `initialize`".into());
+        }
+        self.initialized.store(true, Ordering::SeqCst);
+        Ok(())
+    }
+}