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/src/codex_message_processor.rs

@@ -5465,8 +5465,8 @@ impl CodexMessageProcessor {
                                         PluginSource::Local { path }
                                     }
                                 },
-                                install_policy: plugin.install_policy.map(Into::into),
-                                auth_policy: plugin.auth_policy.map(Into::into),
+                                install_policy: plugin.install_policy.into(),
+                                auth_policy: plugin.auth_policy.into(),
                                 interface: plugin.interface.map(|interface| PluginInterface {
                                     display_name: interface.display_name,
                                     short_description: interface.short_description,
@@ -5719,7 +5719,7 @@ impl CodexMessageProcessor {
                     .send_response(
                         request_id,
                         PluginInstallResponse {
-                            auth_policy: result.auth_policy.map(Into::into),
+                            auth_policy: result.auth_policy.into(),
                             apps_needing_auth,
                         },
                     )