feat: add fixture corpus and bump version to 0.5.6

[pankaj28843]

Summary

• add per-source golden fixtures for full-page HTML to article extraction
• add black-box fixture corpus test comparing extracted vs expected inner text (whitespace-insensitive)
• improve extraction candidate selection/sanitization and bump version to 0.5.6

Validation

• uv run ruff format . && uv run ruff check --fix .
• timeout 60 uv run pytest -v

[codecov]

Codecov Report

❌ Patch coverage is 91.17647% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.61%. Comparing base (3be42f3) to head (ccb65ef).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/article_extractor/extractor.py	90.00%	3 Missing and 4 partials ⚠️
src/article_extractor/content_sanitizer.py	91.30%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
- Coverage   99.66%   99.61%   -0.05%     
==========================================
  Files          31       31              
  Lines        3547     3637      +90     
  Branches      491      509      +18     
==========================================
+ Hits         3535     3623      +88     
- Misses          5        6       +1     
- Partials        7        8       +1     

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