feat(tests): add comprehensive unit tests for core modules

[chicogong]

Summary

Add comprehensive unit tests for previously untested core modules, significantly increasing test coverage.

New Test Files

File	Module	Tests	Description
test_audio_converter.cpp	AudioConverter	19	Int16/float conversion, resampling, stereo-to-mono
test_vad_segmenter.cpp	VADSegmenter	16	Speech detection, segmentation, callbacks, presets
test_rnnoise_processor.cpp	RNNoiseProcessor	21	Initialization, processing, VAD, reset, edge cases
test_logger.cpp	Logger	22	Logging functions, macros, edge cases

Test Results

[==========] Running 133 tests from 7 test suites.
[  PASSED  ] 132 tests.
[  SKIPPED ] 1 test (EdgeCase_ProcessBeforeInit - undefined behavior)

Coverage Improvement

• Before: 55 tests (WAV, FLAC, SignalGenerator)
• After: 133 tests (+78 new tests)
• Increase: 142% more tests

Test Plan

• All new tests pass locally
• Existing tests unaffected
• Python tests pass (6/13, 7 skipped due to module not installed)
• CI/CD workflow already configured

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