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.

sdk/python/src/codex_app_server/models.py

@@ -95,3 +95,5 @@ class ServerInfo(BaseModel):
 class InitializeResponse(BaseModel):
     serverInfo: ServerInfo | None = None
     userAgent: str | None = None
+    platformFamily: str | None = None
+    platformOs: str | None = None