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/config.schema.json

@@ -794,9 +794,6 @@
     "NetworkToml": {
       "additionalProperties": false,
       "properties": {
-        "admin_url": {
-          "type": "string"
-        },
         "allow_local_binding": {
           "type": "boolean"
         },
@@ -818,9 +815,6 @@
         "dangerously_allow_all_unix_sockets": {
           "type": "boolean"
         },
-        "dangerously_allow_non_loopback_admin": {
-          "type": "boolean"
-        },
         "dangerously_allow_non_loopback_proxy": {
           "type": "boolean"
         },