Improve generated code in JSON class

[gcatanese]

Improve the generated JSON.java:

• make variables final (where applicable)
• make constructor private (it is a Singleton)
• replace explicit argument with <>
• set correctly ALLOW_COERCION_OF_SCALARS

[gemini-code-assist]

Summary of Changes

Hello @gcatanese, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on improving the generated JSON class by enhancing its structure and configuration, alongside a minor code simplification in AbstractOpenApiSchema. The changes aim to enforce better design patterns, correct serialization/deserialization behavior, and modernize Java syntax in the generated code.

Highlights

• JSON.java Configuration & Structure: The JSON() constructor was made private to enforce the Singleton pattern. Crucially, the ALLOW_COERCION_OF_SCALARS setting for the ObjectMapper was corrected from false to true and is now properly applied to the mapper instance, which impacts how scalar values are handled during deserialization.
• Code Modernization & Immutability: The diamond operator (<>) was applied to HashSet and HashMap instantiations for improved readability and conciseness. Additionally, modelDiscriminators and modelDescendants fields were marked as final to promote immutability.
• Code Simplification: The isNullable() method in AbstractOpenApiSchema.mustache was refactored for a more concise boolean return statement.

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[gemini-code-assist]

Code Review

This pull request introduces several good improvements to the generated JSON.java class. Key changes include making the constructor private to enforce a singleton pattern, marking collection fields as final, using the diamond operator for cleaner generic type instantiation, and fixing a bug in the configuration of the Jackson ObjectMapper. The code is clearer and more robust as a result. I have one minor suggestion to improve consistency in the use of the diamond operator.