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

@@ -449,6 +449,10 @@ pub struct SkillInterface {
     pub icon_small: Option<AbsolutePathBuf>,
     #[ts(optional)]
     pub icon_large: Option<AbsolutePathBuf>,
+    /// Remote small icon URL from the plugin catalog.
+    pub icon_small_url: Option<String>,
+    /// Remote large icon URL from the plugin catalog.
+    pub icon_large_url: Option<String>,
     #[ts(optional)]
     pub brand_color: Option<String>,
     #[ts(optional)]
@@ -908,6 +912,8 @@ impl From<CoreSkillInterface> for SkillInterface {
             default_prompt: value.default_prompt,
             icon_small: value.icon_small,
             icon_large: value.icon_large,
+            icon_small_url: None,
+            icon_large_url: None,
         }
     }
 }