🧹 [Code Health] Refactor JSON serialization tests to use generic helper

[ffalcinelli]

🎯 What: Replaced repetitive serialization and deserialization blocks in src/models.rs tests with a generic test_serialize_deserialize<T> helper.
💡 Why: Reduces boilerplate, makes the tests DRY, and improves readability and maintainability of the test suite.
✅ Verification: Ran cargo test to confirm all 51 tests successfully pass, including the updated model unit tests.
✨ Result: A cleaner and more concise test module in src/models.rs without changing any runtime behavior.

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PR created automatically by Jules for task 5421246117952995722 started by @ffalcinelli

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.75%. Comparing base (68e538e) to head (4998544).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #58   +/-   ##
=======================================
  Coverage   89.75%   89.75%           
=======================================
  Files          38       38           
  Lines        2489     2489           
=======================================
  Hits         2234     2234           
  Misses        255      255           

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