add @plugin mentions (#13510)

## Note-- added plugin mentions via @, but that conflicts with file
mentions

depends and builds upon #13433.

- introduces explicit `@plugin` mentions. this injects the plugin's mcp
servers, app names, and skill name format into turn context as a dev
message.
- we do not yet have UI for these mentions, so we currently parse raw
text (as opposed to skills and apps which have UI chips, autocomplete,
etc.) this depends on a `plugins/list` app-server endpoint we can feed
the UI with, which is upcoming
- also annotate mcp and app tool descriptions with the plugin(s) they
come from. this gives the model a first class way of understanding what
tools come from which plugins, which will help implicit invocation.

### Tests
Added and updated tests, unit and integration. Also confirmed locally a
raw `@plugin` injects the dev message, and the model knows about its
apps, mcps, and skills.

codex-rs/chatgpt/src/connectors.rs

@@ -446,6 +446,7 @@ fn directory_app_to_app_info(app: DirectoryApp) -> AppInfo {
         install_url: None,
         is_accessible: false,
         is_enabled: true,
+        plugin_display_names: Vec::new(),
     }
 }
 
@@ -483,6 +484,7 @@ mod tests {
             install_url: None,
             is_accessible: false,
             is_enabled: true,
+            plugin_display_names: Vec::new(),
         }
     }
 
@@ -540,6 +542,7 @@ mod tests {
             install_url: Some(connector_install_url(id, id)),
             is_accessible,
             is_enabled: true,
+            plugin_display_names: Vec::new(),
         }
     }