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/state.rs

@@ -21,7 +21,6 @@ pub struct NetworkProxyConstraints {
     pub mode: Option<NetworkMode>,
     pub allow_upstream_proxy: Option<bool>,
     pub dangerously_allow_non_loopback_proxy: Option<bool>,
-    pub dangerously_allow_non_loopback_admin: Option<bool>,
     pub dangerously_allow_all_unix_sockets: Option<bool>,
     pub allowed_domains: Option<Vec<String>>,
     pub denied_domains: Option<Vec<String>>,
@@ -41,7 +40,6 @@ pub struct PartialNetworkConfig {
     pub mode: Option<NetworkMode>,
     pub allow_upstream_proxy: Option<bool>,
     pub dangerously_allow_non_loopback_proxy: Option<bool>,
-    pub dangerously_allow_non_loopback_admin: Option<bool>,
     pub dangerously_allow_all_unix_sockets: Option<bool>,
     #[serde(default)]
     pub allowed_domains: Option<Vec<String>>,
@@ -149,25 +147,6 @@ pub fn validate_policy_against_constraints(
         },
     )?;
 
-    let allow_non_loopback_admin = constraints.dangerously_allow_non_loopback_admin;
-    validate(
-        config.network.dangerously_allow_non_loopback_admin,
-        move |candidate| match allow_non_loopback_admin {
-            Some(true) | None => Ok(()),
-            Some(false) => {
-                if *candidate {
-                    Err(invalid_value(
-                        "network.dangerously_allow_non_loopback_admin",
-                        "true",
-                        "false (disabled by managed config)",
-                    ))
-                } else {
-                    Ok(())
-                }
-            }
-        },
-    )?;
-
     let allow_non_loopback_proxy = constraints.dangerously_allow_non_loopback_proxy;
     validate(
         config.network.dangerously_allow_non_loopback_proxy,