fix: handle JSON object keys containing escape sequences

[pamburus]

Summary

• Nested object keys containing escape sequences caused a panic during formatting
• Such keys are now parsed and displayed correctly without performance degradation for the common case

---

Closes #1313

[codecov]

Codecov Report

❌ Patch coverage is 94.91525% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.05%. Comparing base (95db804) to head (b45ce45).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/model.rs	94.91%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1386      +/-   ##
==========================================
- Coverage   90.05%   90.05%   -0.01%     
==========================================
  Files          72       72              
  Lines       12285    12322      +37     
==========================================
+ Hits        11063    11096      +33     
- Misses       1222     1226       +4     

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