fix(merge): validators mutate the caller's options object

[rajanpanth]

Diagnosed and fixed with AI assistance (Claude). merge() wrote defaults into the passed-in options object (also throwing on a frozen object); see commit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (cdb7daf) to head (0edaa40).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2848   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2599      2598    -1     
  Branches       658       658           
=========================================
- Hits          2599      2598    -1     

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[rajanpanth]

Friendly follow-up on this PR 👋

When you have a moment, could you please take a look? I’m happy to update anything if needed.

Thanks!