refactor: remove proxy admin endpoint (#13687)

## Summary
- delete the network proxy admin server and its runtime listener/task
plumbing
- remove the admin endpoint config, runtime, requirement, protocol,
schema, and debug-surface fields
- update proxy docs to reflect the remaining HTTP and SOCKS listeners
only

codex-rs/core/src/codex.rs

@@ -1017,7 +1017,6 @@ impl Session {
             SessionNetworkProxyRuntime {
                 http_addr: proxy.http_addr().to_string(),
                 socks_addr: proxy.socks_addr().to_string(),
-                admin_addr: proxy.admin_addr().to_string(),
             }
         };
         Ok((network_proxy, session_network_proxy))