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/v2/ConfigRequirementsReadResponse.json

@@ -132,12 +132,6 @@
             "null"
           ]
         },
-        "dangerouslyAllowNonLoopbackAdmin": {
-          "type": [
-            "boolean",
-            "null"
-          ]
-        },
         "dangerouslyAllowNonLoopbackProxy": {
           "type": [
             "boolean",