Add task mentions to the TUI composer (#40315)

## What changed

- Include matching Codex tasks in the `@` mention popup when the active thread supports task tools, prioritizing tasks from the current working directory.
- Submit selected tasks as bounded live thread references so Codex can load their contents with `read_thread`.
- Preserve task mentions across composer history and thread start, resume, and fork flows.

## Testing

- Add coverage for task search and capability propagation, popup selection and rendering, submission encoding, history round-trips, and reference validation and limits.

GitOrigin-RevId: 17502be91bf6d4b01745668829eb69fb318cb5b9

codex-rs/tui/src/chatwidget.rs

@@ -58,7 +58,7 @@ use crate::git_action_directives::parse_assistant_markdown;
 use crate::legacy_core::config::Config;
 use crate::legacy_core::config::PermissionProfileSnapshot;
 use crate::mention_codec::LinkedMention;
-use crate::mention_codec::encode_history_mentions;
+use crate::mention_codec::encode_history_mentions_at_elements;
 use crate::model_catalog::ModelCatalog;
 use crate::multi_agents;
 use crate::multi_agents::AgentMetadata;