Add reusable offline integration test harness

[Copilot]

Summary

• add strict fixture-driven replay adapters for known URLs, search, website crawlers, redirects, and LLM responses
• add an offline collection-to-extraction integration scenario with synthetic documents
• block unexpected network connections and validate every expected interaction is consumed
• document authoring and validating offline integration scenarios

Validation

• Python compilation passed for the changed test modules
• changed files passed secret scanning
• CodeQL skipped because changes are test-only and documentation-only
• pytest and Ruff could not run because Pixi was unavailable and sandbox DNS prevented installation

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.65%. Comparing base (8e6195f) to head (3ebe5df).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #549      +/-   ##
==========================================
+ Coverage   65.09%   65.65%   +0.56%     
==========================================
  Files          78       78              
  Lines        7492     7492              
  Branches      754      754              
==========================================
+ Hits         4877     4919      +42     
+ Misses       2454     2404      -50     
- Partials      161      169       +8     

Flag	Coverage Δ
unittests	65.65% <ø> (+0.56%)	⬆️

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