Remove stdio transport from exec server (#15119)

Summary
- delete the deprecated stdio transport plumbing from the exec server
stack
- add a basic `exec_server()` harness plus test utilities to start a
server, send requests, and await events
- refresh exec-server dependencies, configs, and documentation to
reflect the new flow

Testing
- Not run (not requested)

---------

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

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

@@ -1,20 +1,18 @@
 use clap::Parser;
-use codex_exec_server::ExecServerTransport;
 
 #[derive(Debug, Parser)]
 struct ExecServerArgs {
-    /// Transport endpoint URL. Supported values: `ws://IP:PORT` (default),
-    /// `stdio://`.
+    /// Transport endpoint URL. Supported values: `ws://IP:PORT` (default).
     #[arg(
         long = "listen",
         value_name = "URL",
-        default_value = ExecServerTransport::DEFAULT_LISTEN_URL
+        default_value = codex_exec_server::DEFAULT_LISTEN_URL
     )]
-    listen: ExecServerTransport,
+    listen: String,
 }
 
 #[tokio::main]
 async fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
     let args = ExecServerArgs::parse();
-    codex_exec_server::run_main_with_transport(args.listen).await
+    codex_exec_server::run_main_with_listen_url(&args.listen).await
 }