app-server: Add platform os and family to init response (#14527)

This allows the client to pick os-specific behavior while interacting
with the app server, e.g. to use proper path separators.

codex-rs/app-server-protocol/schema/json/v1/InitializeResponse.json

@@ -1,11 +1,21 @@
 {
   "$schema": "http://json-schema.org/draft-07/schema#",
   "properties": {
+    "platformFamily": {
+      "description": "Platform family for the running app-server target, for example `\"unix\"` or `\"windows\"`.",
+      "type": "string"
+    },
+    "platformOs": {
+      "description": "Operating system for the running app-server target, for example `\"macos\"`, `\"linux\"`, or `\"windows\"`.",
+      "type": "string"
+    },
     "userAgent": {
       "type": "string"
     }
   },
   "required": [
+    "platformFamily",
+    "platformOs",
     "userAgent"
   ],
   "title": "InitializeResponse",