Expose plugin installation interstitial requirements (#33896)

## What changed

- Add nullable `mustShowInstallationInterstitial` metadata to app-server
  `PluginSummary` responses.
- Preserve the remote service value through catalog, installed-plugin cache,
  list, read, and share-list paths.
- Return `null` for local plugins and remote responses that omit the field so
  clients can fail closed when the policy is unavailable.

## Testing

- Cover `true`, `false`, and missing values across remote summary paths and
  app-server plugin endpoints.

GitOrigin-RevId: db79702ee2bfa800a9910d64d933bcabecb3ce70

codex-rs/app-server-protocol/src/protocol/v2/plugin.rs

@@ -625,6 +625,8 @@ pub struct PluginSummary {
     pub enabled: bool,
     pub install_policy: PluginInstallPolicy,
     pub install_policy_source: Option<PluginInstallPolicySource>,
+    #[serde(default)]
+    pub must_show_installation_interstitial: Option<bool>,
     pub auth_policy: PluginAuthPolicy,
     /// Availability state for installing and using the plugin.
     #[serde(default)]