Enrich app/read connector metadata (#34417)

## What changed

- Add `iconUrlDark`, `distributionChannel`, `installUrl`, and
  `pluginDisplayNames` to the experimental `app/read` connector metadata.
- Populate plugin display names from enabled plugins without starting their MCP
  servers.
- Accept both dark-icon field spellings from the batch API and keep the new
  upstream fields optional for compatibility with older responses.

## Testing

- Cover the serialized response fields, legacy batch responses, and plugin
  display-name lookup without MCP startup.

GitOrigin-RevId: 68601a104de839c0f4f4621d33b0e4d08ad88c64

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

@@ -202,6 +202,11 @@ pub struct ConnectorMetadata {
     pub name: String,
     pub description: Option<String>,
     pub icon_url: Option<String>,
+    pub icon_url_dark: Option<String>,
+    pub distribution_channel: Option<String>,
+    pub install_url: Option<String>,
+    #[serde(default)]
+    pub plugin_display_names: Vec<String>,
     pub tool_summaries: Option<Vec<AppToolSummary>>,
 }