fix: add score_threshold support to BestFirstCrawlingStrategy

[nightcityblade]

Summary

Add score_threshold parameter to BestFirstCrawlingStrategy, matching the existing behavior in BFSDeepCrawlStrategy and DFSDeepCrawlStrategy.

Fixes #1801

List of files changed and why

crawl4ai/deep_crawling/bff_strategy.py — Added score_threshold parameter to __init__ (defaulting to -infinity for backward compatibility) and added threshold check in the link discovery loop to skip URLs scoring below the threshold.

How Has This Been Tested?

• Code review against BFS/DFS implementations to ensure consistent behavior
• Default value of -infinity ensures no behavioral change for existing users

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

[unclecode]

Thanks @nightcityblade — merged into develop, will be in the next release. Good addition to keep BestFirstCrawlingStrategy consistent with BFS/DFS. You've been added to CONTRIBUTORS.md.