KF-31 Add tests for USimpleString

[belyshevdenis]

relied to and can be merged with #35

[Copilot]

Pull Request Overview

This PR adds comprehensive test coverage for the USimpleString class, implementing over 80 test cases that verify all public methods and operators. It also adds minor documentation improvements to clarify comparison behavior in the header file.

• Adds a complete test suite for USimpleString with edge cases and Unicode support
• Tests both empty and non-empty string scenarios across all methods
• Includes documentation comments for comparison operators and return codes

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
test/USimpleStringTest.cpp	New comprehensive test file covering all USimpleString methods with empty and non-empty string scenarios
test/CMakeLists.txt	Adds the new test file to the build configuration
include/kf/USimpleString.h	Adds documentation comments explaining comparison return codes and operator behavior

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