test: strengthen decision assertions

[rlippmann]

What changed

• Strengthen the shared Decision assertion helper to check supplied public fields.
• Update primary Engine tests to assert changed, failure types, rejected directives, ordered repairs, and messages.

Why

• Prevent tests from passing when structured Decision data is incorrect while the kind or message remains unchanged.

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (142b655) to head (ce4e11d).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #271   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          783       783           
=========================================
  Hits           783       783           

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