Use stringstreams for the error messages in the validators; fixes #475

[liuzicheng1987]

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[gemini-code-assist]

Summary of Changes

Hello @liuzicheng1987, 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!

I've refactored the error message generation within the validator classes to utilize std::stringstream instead of std::to_string and direct string concatenation. This change aims to improve the clarity and potentially the efficiency of how error messages are constructed. Additionally, I've introduced a new test case to ensure that validation with fields continues to function as expected.

Highlights

• Error Message Refactoring: Switched from using std::to_string and string concatenation to std::stringstream for building detailed error messages in validator classes like EqualTo, Minimum, Maximum, etc.
• Code Readability and Maintainability: The adoption of std::stringstream enhances the readability and maintainability of the error message construction logic, especially for more complex messages.
• New Test Coverage: Added a new test file, tests/json/test_validation_with_fields.cpp, to specifically cover scenarios involving validation with fields, ensuring the robustness of the validator system.

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[gemini-code-assist]

Code Review

This pull request refactors the validation error message generation in rfl::comparisons.hpp to use std::stringstream instead of string concatenation, which is a good improvement for performance and type safety. A new test file is also added to cover validation on struct fields. My review includes a few suggestions for the new test file to improve its correctness and remove redundant code. I've also suggested an alternative approach using std::format for string building in comparisons.hpp, which could be even more efficient and readable if your toolchain supports C++20.