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/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -7261,9 +7261,6 @@
     },
     "SessionNetworkProxyRuntime": {
       "properties": {
-        "admin_addr": {
-          "type": "string"
-        },
         "http_addr": {
           "type": "string"
         },
@@ -7272,7 +7269,6 @@
         }
       },
       "required": [
-        "admin_addr",
         "http_addr",
         "socks_addr"
       ],
@@ -11023,12 +11019,6 @@
               "null"
             ]
           },
-          "dangerouslyAllowNonLoopbackAdmin": {
-            "type": [
-              "boolean",
-              "null"
-            ]
-          },
           "dangerouslyAllowNonLoopbackProxy": {
             "type": [
               "boolean",