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/src/tasks/user_shell.rs

@@ -188,6 +188,7 @@ pub(crate) async fn execute_user_shell_command(
         .emit_turn_item_started(
             turn_context.as_ref(),
             &TurnItem::CommandExecution(CommandExecutionItem {
+                model_context: None,
                 id: call_id.clone(),
                 plugin_id: None,
                 script_path: None,
@@ -271,6 +272,7 @@ pub(crate) async fn execute_user_shell_command(
                 .emit_turn_item_completed(
                     turn_context.as_ref(),
                     TurnItem::CommandExecution(CommandExecutionItem {
+                        model_context: None,
                         id: call_id,
                         plugin_id: None,
                         script_path: None,
@@ -296,6 +298,7 @@ pub(crate) async fn execute_user_shell_command(
                 .emit_turn_item_completed(
                     turn_context.as_ref(),
                     TurnItem::CommandExecution(CommandExecutionItem {
+                        model_context: None,
                         id: call_id.clone(),
                         plugin_id: None,
                         script_path: None,
@@ -341,6 +344,7 @@ pub(crate) async fn execute_user_shell_command(
                 .emit_turn_item_completed(
                     turn_context.as_ref(),
                     TurnItem::CommandExecution(CommandExecutionItem {
+                        model_context: None,
                         id: call_id,
                         plugin_id: None,
                         script_path: None,