Enforce JSONSerializer string output contract

[foreverest]

Summary

• add a post-JSON.stringify check so JSONSerializer now throws SerializerError unless it returns a string
• surface the new guard via a regression test that expects undefined serialization to throw

Testing

• Not run (not requested)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (675d379) to head (d9290cc).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #21   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          193       200    +7     
  Branches        38        42    +4     
=========================================
+ Hits           193       200    +7     

Flag	Coverage Δ
node20.x	100.00% <100.00%> (ø)
node22.x	100.00% <100.00%> (ø)
node24.x	100.00% <100.00%> (ø)

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