Add root turn IDs to turn and tool analytics (#40486)

## Why

Analytics need to associate subagent activity with the top-level turn that
caused it, while avoiding a stale association when steering makes that origin
ambiguous.

## What changed

- Add `root_turn_id` to turn and tool-item analytics events.
- Read the root from live turn metadata when each event is built, reporting
  `null` when the root is unknown or ambiguous.
- Propagate the metadata through control-tool and code-mode completion facts so
  correlated and background tool events retain the correct root.

## Testing

- Cover root IDs for top-level turns, subagent turns, correlated code-mode
  calls, and tool events emitted after turn completion.
- Verify that queued and subsequent events report `null` after the live root is
  invalidated.

GitOrigin-RevId: 42fd76541c3c716cec74787d47b50adb2d22e6d2

codex-rs/analytics/src/client_tests.rs

@@ -194,6 +194,7 @@ fn sample_mcp_tool_call_event(thread_id: &str, plugin_id: Option<&str>) -> Track
                 thread_id: thread_id.to_string(),
                 session_id: format!("session-{thread_id}"),
                 turn_id: "turn-1".to_string(),
+                root_turn_id: None,
                 item_id: format!("item-{thread_id}"),
                 cell_id: None,
                 parent_call_id: None,