[FLINK-40376][formats] Support VARIANT type in flink-json converters

[ferenc-csaky]

What is the purpose of the change

Integrate VARIANT handling to flink-json converters.

Brief change log

• Added VARIANT deserialization support to both JSON tree and streaming converters.
• Added VARIANT serialization support in the JSON row-data converter.

Verifying this change

This change added tests and can be verified as follows:

• Added round-trip tests for nested VARIANT objects and arrays through both JSON deserializers.
• Added coverage for scalar VARIANT values and variants nested in arrays, maps, and rows.
• Added tests for duplicate JSON keys, JSON-null behavior, and rejection of non-finite variant values during serialization.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

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Was generative AI tooling used to co-author this PR?

• Yes

Generated-by: openai/gpt-5.6-terra

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CI report:

• d3515c9 Azure: SUCCESS

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