[apps] Thread structured icon assets through app list (#29889)

## Summary

- Add `iconAssets` and `iconDarkAssets` to the app-list protocol.
- Preserve structured icons through directory merging and the connector,
app-
  server, and TUI boundaries.
- Keep legacy logo URLs unchanged as compatibility fallbacks.
- Update generated protocol schemas and TypeScript types.

codex-rs/tools/src/request_plugin_install_tests.rs

@@ -11,6 +11,8 @@ fn build_request_plugin_install_elicitation_request_uses_expected_shape() {
         description: Some("Plan events and schedules.".to_string()),
         logo_url: None,
         logo_url_dark: None,
+        icon_assets: None,
+        icon_dark_assets: None,
         distribution_channel: None,
         branding: None,
         app_metadata: None,
@@ -124,6 +126,8 @@ fn build_request_plugin_install_meta_uses_expected_shape() {
         description: None,
         logo_url: None,
         logo_url_dark: None,
+        icon_assets: None,
+        icon_dark_assets: None,
         distribution_channel: None,
         branding: None,
         app_metadata: None,
@@ -165,6 +169,8 @@ fn verified_connector_install_completed_requires_accessible_connector() {
         description: None,
         logo_url: None,
         logo_url_dark: None,
+        icon_assets: None,
+        icon_dark_assets: None,
         distribution_channel: None,
         branding: None,
         app_metadata: None,
@@ -193,6 +199,8 @@ fn all_requested_connectors_picked_up_requires_every_expected_connector() {
         description: None,
         logo_url: None,
         logo_url_dark: None,
+        icon_assets: None,
+        icon_dark_assets: None,
         distribution_channel: None,
         branding: None,
         app_metadata: None,