Fix macOS voice meter source assertions

[basnijholt]

Summary

• Update macOS voice meter source-shape assertions for the AVAudioEngine spectrum implementation
• Keep the no-audio-file guarantee by asserting AVAudioRecorder is absent

Test Plan

• uv run pytest tests/test_macos_app.py::test_macos_app_voice_level_meter_uses_live_audio_engine_without_saving_audio tests/test_macos_app.py::test_macos_app_voice_level_meter_smooths_fast_meter_changes -q
• uv run --all-extras pytest
• pre-commit run --all-files
• swift test --package-path macos/AgentCLI --enable-xctest (local build completed; XCTest unavailable with Command Line Tools xcrun PlatformPath error)