fix(models): support float timestamps in CrawlStats

Modify CrawlStats class to handle both datetime and float timestamp formats for start_time and end_time fields. This change improves compatibility with different time formats while maintaining existing functionality.

Other minor changes:
- Add datetime import in async_dispatcher
- Update JsonElementExtractionStrategy kwargs handling

No breaking changes.

crawl4ai/async_dispatcher.py

@@ -13,7 +13,7 @@ from rich.live import Live
 from rich.table import Table
 from rich.console import Console
 from rich import box
-from datetime import timedelta
+from datetime import timedelta, datetime
 from collections.abc import AsyncGenerator
 import time
 import psutil