Make environment context date and timezone configurable (#52164)

## What changed

Add `include_environment_context_time`, enabled by default. Setting it to `false` omits `<current_date>` and `<timezone>` while retaining the rest of the environment context, and skips the clock read used to populate the date.

## Testing

Extend configuration, session context, and external time provider tests to cover enabled and disabled time fields, context updates across turns, and current-time reminder behavior.

GitOrigin-RevId: 5071ce1701c044b680624d0cdbba7e0bc2b5bbe6

codex-rs/thread-manager-sample/src/main.rs

@@ -280,6 +280,7 @@ async fn new_config(
         cloud_skill_enabled: false,
         orchestrator_mcp_enabled: false,
         include_environment_context: false,
+        include_environment_context_time: true,
         compact_prompt: None,
         notify: None,
         tui_notifications: TuiNotificationSettings::default(),