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/exec/tests/event_processor_with_json_output.rs

@@ -169,6 +169,7 @@ fn turn_started_emits_turn_started_event() {
 fn command_execution_started_and_completed_translate_to_thread_events() {
     let mut processor = EventProcessorWithJsonOutput::new(/*last_message_path*/ None);
     let command_item = ThreadItem::CommandExecution {
+        model_context: None,
         id: "cmd-1".to_string(),
         command: "ls".to_string(),
         cwd: test_path_buf("/tmp/project").abs().into(),
@@ -211,6 +212,7 @@ fn command_execution_started_and_completed_translate_to_thread_events() {
     let completed = processor.collect_thread_events(ServerNotification::ItemCompleted(
         ItemCompletedNotification {
             item: ThreadItem::CommandExecution {
+                model_context: None,
                 id: "cmd-1".to_string(),
                 command: "ls".to_string(),
                 cwd: test_path_buf("/tmp/project").abs().into(),
@@ -1348,6 +1350,7 @@ fn turn_completion_reconciles_started_items_from_turn_items() {
     let started =
         processor.collect_thread_events(ServerNotification::ItemStarted(ItemStartedNotification {
             item: ThreadItem::CommandExecution {
+                model_context: None,
                 id: "cmd-1".to_string(),
                 command: "ls".to_string(),
                 cwd: test_path_buf("/tmp/project").abs().into(),
@@ -1390,6 +1393,7 @@ fn turn_completion_reconciles_started_items_from_turn_items() {
                 id: "turn-1".to_string(),
                 items_view: codex_app_server_protocol::TurnItemsView::Full,
                 items: vec![ThreadItem::CommandExecution {
+                    model_context: None,
                     id: "cmd-1".to_string(),
                     command: "ls".to_string(),
                     cwd: test_path_buf("/tmp/project").abs().into(),