fix: keep fallback_fetch_function and other fields in CrawlerRunConfig.clone()

[Parsiffall1]

Summary

Fixes #2379

CrawlerRunConfig.clone() rebuilds the config from to_dict(), and to_dict() did not include fallback_fetch_function. BFS, DFS and BestFirst deep crawls clone the run config for every batch (config.clone(deep_crawl_strategy=None, stream=...)), so the fallback fetcher was dropped and never used for deep-crawled pages, while the same page fetched with arun() / arun_many() did use it.

check_cache_freshness, cache_validation_timeout and virtual_scroll_config were missing from to_dict() for the same reason and were also reset to their defaults on clone. This adds the four keys to to_dict(); clone() keeps working as before and still lets callers override any of them through kwargs.

List of files changed and why

• crawl4ai/async_configs.py: add fallback_fetch_function, check_cache_freshness, cache_validation_timeout and virtual_scroll_config to CrawlerRunConfig.to_dict().
• tests/unit/test_run_config_clone_fields.py: new tests that clone a config the way deep crawl strategies do and check these fields survive, and that fallback_fetch_function can still be overridden.

How Has This Been Tested?

• pytest tests/unit/test_run_config_clone_fields.py: 3 passed; 2 of them fail on develop without the change.
• pytest tests/unit: the same set of failures before and after the change (they need network/browser access locally); no new failures.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added/updated unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes