Expose remote skill icon URLs through app server (#35012)

## What changed

- Add nullable `iconSmallUrl` and `iconLargeUrl` fields to the v2
  `SkillInterface` protocol and generated schemas.
- Populate the fields from remote plugin catalog metadata while returning null
  values for local skill interfaces.
- Document the new `plugin/read` response fields.

## Testing

- Extend the remote `plugin/read` test to verify both icon URLs are returned.

GitOrigin-RevId: 0a631a0121627d944a84e542728a506823ff2738

codex-rs/tui/src/bottom_pane/chat_composer.rs

@@ -7329,6 +7329,8 @@ mod tests {
                 short_description: None,
                 icon_small: None,
                 icon_large: None,
+                icon_small_url: None,
+                icon_large_url: None,
                 brand_color: None,
                 default_prompt: None,
             }),
@@ -7444,6 +7446,8 @@ mod tests {
                         short_description: None,
                         icon_small: None,
                         icon_large: None,
+                        icon_small_url: None,
+                        icon_large_url: None,
                         brand_color: None,
                         default_prompt: None,
                     }),