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/src/protocol/v2.rs

@@ -3087,8 +3087,8 @@ pub struct PluginSummary {
     pub source: PluginSource,
     pub installed: bool,
     pub enabled: bool,
-    pub install_policy: Option<PluginInstallPolicy>,
-    pub auth_policy: Option<PluginAuthPolicy>,
+    pub install_policy: PluginInstallPolicy,
+    pub auth_policy: PluginAuthPolicy,
     pub interface: Option<PluginInterface>,
 }
 
@@ -3149,7 +3149,7 @@ pub struct PluginInstallParams {
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
 pub struct PluginInstallResponse {
-    pub auth_policy: Option<PluginAuthPolicy>,
+    pub auth_policy: PluginAuthPolicy,
     pub apps_needing_auth: Vec<AppSummary>,
 }