chore: use AVAILABLE and ON_INSTALL as default plugin install and auth policies (#14407)

make `AVAILABLE` the default plugin installPolicy when unset in
`marketplace.json`. similarly, make `ON_INSTALL` the default authPolicy.

this means, when unset, plugins are available to be installed (but not
auto-installed), and the contained connectors will be authed at
install-time.

updated tests.

codex-rs/app-server-protocol/schema/json/v2/PluginInstallResponse.json

@@ -45,18 +45,12 @@
       "type": "array"
     },
     "authPolicy": {
-      "anyOf": [
-        {
-          "$ref": "#/definitions/PluginAuthPolicy"
-        },
-        {
-          "type": "null"
-        }
-      ]
+      "$ref": "#/definitions/PluginAuthPolicy"
     }
   },
   "required": [
-    "appsNeedingAuth"
+    "appsNeedingAuth",
+    "authPolicy"
   ],
   "title": "PluginInstallResponse",
   "type": "object"