fix: update test class stubs and remove PowerMock for Mockito 5 compatibility

- Removed PowerMock dependencies from pom.xml as it's incompatible with modern Mockito (v5+)
- Updated pom.xml to use JUnit 5 (`junit-jupiter`) and `mockito-junit-jupiter` for clean integration
- Replaced incorrect stubbing of long-returning methods (`getDepth()`, `getFrames()`, `getChannels()`)
  in test class with proper `long` literals (e.g., `5L` instead of `5`)
- Ensured all Mockito stubs match the exact return types to avoid `WrongTypeOfReturnValue` errors
- Confirmed tests now run under JUnit 5 with `MockitoExtension` and no legacy dependencies