Implement struct-based JSON serialization with visitor pattern

[mfaferek93]

Summary

Current JSON response building uses manual field-by-field assignment (data_result["status"] = "data"). This is error-prone and requires updates in multiple places when response structure changes.

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Proposed solution

Define response structs (e.g., TopicSampleResponse) and implement template-based JSON conversion using visitor pattern. Libraries like nlohmann/json support to_json/from_json ADL customization points.

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Additional context

Reference: data_access_manager.cpp - manual JSON construction in get_topic_sample_with_fallback()