[codex] Preserve ordered dict output by default

[quinnj]

Summary

• Limit default sort_keys=nothing behavior to plain Dict values.
• Preserve iteration order for other AbstractDict implementations, including OrderedDict-style containers.
• Keep explicit sort_keys=true behavior sorting all AbstractDict values.

Root Cause

#442 made the default writer sort every non-JSON.Object AbstractDict. That improved deterministic output for unordered Dict, but it also changed the semantics of ordered dictionary implementations whose iteration order is meaningful.

Validation

• Reproduced JSON 1.4.0 vs 1.5.0 behavior with OrderedCollections.OrderedDict.
• Verified this branch restores insertion-order output for OrderedDict while preserving sort_keys=true.
• Ran julia --startup-file=no --project=. -e 'using Pkg; Pkg.test(; coverage=false)'.

Fixes #447.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.30%. Comparing base (53df2b3) to head (dc22640).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #452   +/-   ##
=======================================
  Coverage   90.29%   90.30%           
=======================================
  Files           7        7           
  Lines        1391     1392    +1     
=======================================
+ Hits         1256     1257    +1     
  Misses        135      135           

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