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/network-proxy/src/lib.rs

@@ -1,6 +1,5 @@
 #![deny(clippy::print_stdout, clippy::print_stderr)]
 
-mod admin;
 mod certs;
 mod config;
 mod http_proxy;