Attribute command and plugin analytics to the invoking model (#45445)

## Why

Command execution and plugin measurement events lack model and reasoning-effort labels. Attribution needs to reflect the step that invoked the command, even when model settings change before a background process finishes.

## What changed

- Add `model_slug` and `reasoning_effort` to command execution and plugin measurement analytics.
- Capture model context from resolved step settings and carry it through execution, approval, Guardian review, and plugin metrics collection.
- Preserve the first command-start model context when subsequent start notifications arrive.
- Keep the carried context out of serialized protocol items and generated schemas.

## Testing

Extend analytics tests to cover model switches before invocation and during background execution, default reasoning effort, Guardian-denied commands, and repeated start notifications retaining the original model context.

GitOrigin-RevId: af90e1c0d39bab625f2e89786085b61a9b96c0ce

codex-rs/thread-store/src/local/rollout_migration/legacy_event.rs

@@ -213,6 +213,7 @@ pub(super) fn completed_item(
         )),
         EventMsg::ExecCommandEnd(event) => Some((
             TurnItem::CommandExecution(CommandExecutionItem {
+                model_context: None,
                 id: event.call_id.clone(),
                 plugin_id: event.plugin_id.clone(),
                 script_path: event.script_path.clone(),