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/server.rs

@@ -4,15 +4,15 @@ mod processor;
 mod transport;
 
 pub(crate) use handler::ExecServerHandler;
-pub use transport::ExecServerTransport;
-pub use transport::ExecServerTransportParseError;
+pub use transport::DEFAULT_LISTEN_URL;
+pub use transport::ExecServerListenUrlParseError;
 
 pub async fn run_main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
-    run_main_with_transport(ExecServerTransport::Stdio).await
+    run_main_with_listen_url(DEFAULT_LISTEN_URL).await
 }
 
-pub async fn run_main_with_transport(
-    transport: ExecServerTransport,
+pub async fn run_main_with_listen_url(
+    listen_url: &str,
 ) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
-    transport::run_transport(transport).await
+    transport::run_transport(listen_url).await
 }