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

@@ -1709,6 +1709,8 @@ pub struct AppInfo {
     /// ```
     #[serde(default = "default_enabled")]
     pub is_enabled: bool,
+    #[serde(default)]
+    pub plugin_display_names: Vec<String>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]