Add exec-server exec RPC implementation (#15090)

Stacked PR 2/3, based on the stub PR.

Adds the exec RPC implementation and process/event flow in exec-server
only.

---------

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

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

@@ -1,5 +1,8 @@
 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 {
@@ -15,3 +18,10 @@ pub struct RemoteExecServerConnectArgs {
     pub connect_timeout: Duration,
     pub initialize_timeout: Duration,
 }
+
+/// Connection-level server events.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum ExecServerEvent {
+    OutputDelta(ExecOutputDeltaNotification),
+    Exited(ExecExitedNotification),
+}