Add exec-server process RPC implementation

Restore the process RPC client/server implementation on top of the initialize-only base.\n\nCo-authored-by: Codex <noreply@openai.com>

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

@@ -0,0 +1,27 @@
+use std::time::Duration;
+
+use crate::protocol::ExecExitedNotification;
+use crate::protocol::ExecOutputDeltaNotification;
+
+/// Connection options for any exec-server client transport.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ExecServerClientConnectOptions {
+    pub client_name: String,
+    pub initialize_timeout: Duration,
+}
+
+/// WebSocket connection arguments for a remote exec-server.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct RemoteExecServerConnectArgs {
+    pub websocket_url: String,
+    pub client_name: String,
+    pub connect_timeout: Duration,
+    pub initialize_timeout: Duration,
+}
+
+/// Connection-level server events.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum ExecServerEvent {
+    OutputDelta(ExecOutputDeltaNotification),
+    Exited(ExecExitedNotification),
+}