refactor(tests): YAML-driven fixtures for preprocessor tests

- cases.yaml: 10 test cases con schema dichiarativo (op, assertions)
- data/: 7 file reali (email_action.html, email_thread.html, email_single.html,
  email_heavy.html, generic_page.html, notes.txt, fallback.txt)
- test_preprocessors.py: parametrize da YAML via test_detect / test_preprocess;
  assertion engine generico (no_html_tags, min_length, compression_ratio,
  metadata_keys, contains, not_contains, content_type)
- requirements.txt: add PyYAML

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/fixtures/preprocessors/data/notes.txt

@@ -0,0 +1,15 @@
+Meeting notes - April 7, 2026
+
+Attendees: Alice, Bob, Charlie
+
+Discussion points:
+- Deploy scheduled for Friday
+- Bug fix for login must be completed by Thursday
+- Review Q1 numbers before EOW
+
+Action items:
+- Alice: fix login bug
+- Bob: prepare deploy checklist
+- Charlie: send Q1 report
+
+Next meeting: April 14, 2026