Honor explicit plugin mentions during MCP startup (#42406)

## Why

Selected plugin MCP servers may be skipped after the optional startup grace
period. When a user explicitly mentions a plugin or MCP server, the turn should
continue waiting so the requested tools are available to the model.

## What changed

- Track explicitly mentioned plugin IDs and MCP servers across later samples,
  steered input, and same-turn restarts.
- Require matching selected-plugin MCP servers to finish startup before
  capturing the tool catalog, while leaving unmentioned servers optional.
- Match `plugin://` identities independently of display names and ignore
  targeting query parameters such as `app` and `browserFamily`.

## Testing

Added coverage for linked and structured plugin mentions, direct and batched
capability discovery, delayed environment attachment, steering, and same-turn
restarts.

GitOrigin-RevId: c509ef8df45df1a7edb0da6afdb2682dc53a4de8

codex-rs/core/src/plugins/mod.rs

@@ -23,6 +23,7 @@ pub(crate) use render::render_explicit_plugin_instructions;
 
 pub(crate) use mentions::build_connector_slug_counts;
 pub(crate) use mentions::collect_explicit_app_ids;
+pub(crate) use mentions::collect_explicit_plugin_ids;
 pub(crate) use mentions::collect_explicit_plugin_mentions;
 pub(crate) use mentions::collect_tool_mentions_from_messages;