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/protocol/src/protocol.rs

@@ -5516,6 +5516,7 @@ mod tests {
             turn_id: "turn-1".into(),
             started_at_ms: 10,
             item: TurnItem::CommandExecution(CommandExecutionItem {
+                model_context: None,
                 id: "exec-1".into(),
                 plugin_id: Some("sample@openai-curated".into()),
                 script_path: Some("scripts/run.py".into()),
@@ -5542,6 +5543,7 @@ mod tests {
             started_at_ms: Some(10),
             completed_at_ms: 20,
             item: TurnItem::CommandExecution(CommandExecutionItem {
+                model_context: None,
                 id: "exec-1".into(),
                 plugin_id: Some("sample@openai-curated".into()),
                 script_path: Some("scripts/run.py".into()),