Fix auth-aware request deduplication

[3em0]

Summary

• include authentication-related request context in the default scheduler fingerprint
• keep non-auth headers behind fp_include_headers to preserve normal deduplication behavior
• add regression tests for Authorization, Cookie, extra_headers, and explicit cookies request context
• update scheduler deduplication docs

Closes #313

Tests

• pytest tests/spiders/test_request.py tests/spiders/test_scheduler.py
• ruff check scrapling/spiders/request.py tests/spiders/test_request.py tests/spiders/test_scheduler.py

[D4Vinci]

Closing as per the issue.