Add unit tests for all source sets

[OneXeor]

Description

Add comprehensive unit tests for common, Android, and iOS source sets.

Scope

• commonTest: Test shared logic and data classes
• androidUnitTest: Test Android-specific implementations
• iosTest: Test iOS-specific implementations

Testing Framework

• kotlin.test for common/multiplatform tests
• Consider MockK or similar for mocking native SDK calls

Areas to Cover

• UXConfig data class validation
• API method parameter validation
• Error handling scenarios