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/core/tests/suite/context_annotations.rs

@@ -76,6 +76,7 @@ async fn first_request_item_types_roles_and_content_annotations() -> Result<()>
         .submit(Op::ApproveGuardianDeniedAction {
             event: GuardianAssessmentEvent {
                 review_reason: None,
+                model_context: None,
                 id: "guardian-review".to_string(),
                 target_item_id: None,
                 plugin_id: None,