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/PluginListResponse.json

@@ -170,14 +170,7 @@
     "PluginSummary": {
       "properties": {
         "authPolicy": {
-          "anyOf": [
-            {
-              "$ref": "#/definitions/PluginAuthPolicy"
-            },
-            {
-              "type": "null"
-            }
-          ]
+          "$ref": "#/definitions/PluginAuthPolicy"
         },
         "enabled": {
           "type": "boolean"
@@ -186,14 +179,7 @@
           "type": "string"
         },
         "installPolicy": {
-          "anyOf": [
-            {
-              "$ref": "#/definitions/PluginInstallPolicy"
-            },
-            {
-              "type": "null"
-            }
-          ]
+          "$ref": "#/definitions/PluginInstallPolicy"
         },
         "installed": {
           "type": "boolean"
@@ -216,8 +202,10 @@
         }
       },
       "required": [
+        "authPolicy",
         "enabled",
         "id",
+        "installPolicy",
         "installed",
         "name",
         "source"