KF-31 Add tests for UStringBuilder

[belyshevdenis]

No description provided.

[Copilot]

Pull Request Overview

This pull request adds comprehensive unit tests for the UStringBuilder class, covering its core functionality including construction, reservation, appending operations, and string manipulation features.

• Adds a new test file UStringBuilderTest.cpp with tests for UStringBuilder functionality
• Integrates the new test file into the CMake build system
• Tests cover default construction, memory reservation, string appending, and content truncation scenarios

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
test/UStringBuilderTest.cpp	Comprehensive test suite for UStringBuilder covering construction, reservation, append operations, and edge cases
test/CMakeLists.txt	Adds UStringBuilderTest.cpp to the test build configuration

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