🧪 [testing improvement] Add tests for JSON stringify_value

[Tcode-Motion]

🎯 What: Added tests for the stringify_value function in stdlib/src/json.rs to address the untested Stringify value rationale.
📊 Coverage: The new tests cover:

• Serialization of simple primitive types (Null, Bool, Int, Float).
• Serialization of Str (strings) with and without escaping quotes.
• Serialization of complex types like List and Map.
• Serialization of nested structures containing both lists and maps.
• Error handling by verifying an error is thrown when attempting to stringify unsupported types (like an empty Tuple).
  ✨ Result: Test coverage for stringify_value in the json.rs standard library module is significantly improved and verifies correct functionality across various data types.

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PR created automatically by Jules for task 1929754128045803723 started by @Tcode-Motion

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