fix: handle encoding errors in sitemap URL seeding gracefully

[Br1an67]

Summary

When a sitemap contains URLs with Unicode characters like zero-width spaces (U+200B), the producer in async_url_seeder.py crashes with a UnicodeEncodeError on Windows (charmap codec), which terminates URL seeding prematurely.

This PR:

1. Adds per-URL error handling in the producer loop so encoding errors skip the problematic URL instead of killing the entire producer
2. Strips zero-width space (U+200B) and BOM (U+FEFF) characters from sitemap URLs during normalization in both _iter_sitemap_content() and _iter_sitemap()

Fixes #1542

List of files changed and why

• crawl4ai/async_url_seeder.py — Added UnicodeEncodeError catch in producer loop; strip invisible Unicode chars in _normalize_loc()

How Has This Been Tested?

Verified that the producer continues processing after encountering URLs with zero-width spaces, and that _normalize_loc correctly strips invisible characters.

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