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/core/src/tools/spec.rs

@@ -3076,6 +3076,7 @@ mod tests {
                         ),
                         connector_id: Some("calendar".to_string()),
                         connector_name: Some("Calendar".to_string()),
+                        plugin_display_names: Vec::new(),
                     },
                 ),
                 (
@@ -3086,6 +3087,7 @@ mod tests {
                         tool: mcp_tool("echo", "Echo", serde_json::json!({"type": "object"})),
                         connector_id: None,
                         connector_name: None,
+                        plugin_display_names: Vec::new(),
                     },
                 ),
             ])),