fix(deep-crawl): count queued next-level URLs against max_pages in BFS

[drakeo338]

Fixes #2338.

Summary

In BFS deep crawl, link_discovery computed the remaining page budget per parent page without counting URLs already queued in next_level by sibling parents. Each parent could queue the full remaining budget, so one level overshot max_pages (14 left with 35 parents of 30 links each queued 490 URLs). The remaining capacity now subtracts len(next_level).

List of files changed and why

• crawl4ai/deep_crawling/bfs_strategy.py - subtract already-queued URLs from remaining capacity
• tests/deep_crawling/test_bfs_max_pages_next_level.py - regression test

How Has This Been Tested?

The new test fails before the change and passes after it.

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

Unticked: only the new test was run, not the full suite; no comments or docs changes were needed.