refactor: optimize json2 write

[MichaelScofield]

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PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

[gemini-code-assist]

Code Review

This pull request refactors the conversion of JSON values into struct vectors by introducing schema-aware conversion helpers (json_variant_to_struct_value_ref and json_variant_to_value_ref) and removing the as_value_ref method from JsonValueRef. It also adds a push_struct_value_ref method to StructVectorBuilder to push struct value references directly, and updates the pipeline transformer to resolve schemas using ConcreteDataType::json2. I have no feedback to provide as there are no review comments.

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