Add Testing Section to Contributing Docs

[trobanga]

Summary

• Add new docs/contributing/testing.md page documenting the multi-layer testing strategy (unit, integration, agent E2E, E2E)
• Add Testing entry to VitePress contributing sidebar in config.mts
• Add cross-reference link from contributing.md to the new testing page

Closes #937

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.30%. Comparing base (6f81d64) to head (b75074f).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1474   +/-   ##
=========================================
  Coverage     99.30%   99.30%           
  Complexity      515      515           
=========================================
  Files           124      124           
  Lines          1877     1877           
  Branches         51       51           
=========================================
  Hits           1864     1864           
  Misses            2        2           
  Partials         11       11           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.