chore: remove dead test code and fix stale comment

tests/test_isr.py:
- Remove unused imports: io, SimpleNamespace, call
- Remove test_mp3_chunks_written_to_file — it mocked connect_stream but
  never called record(), making all the mock scaffolding dead; the actual
  assertion (bytes written to a file) is already covered by
  TestAudioFileWriter and TestGenerateFilename

isr.py:
- Update AudioDevice.backend comment: only the ALSA backend exists now

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

isr.py

@@ -54,7 +54,7 @@ class AudioDevice:
     name: str                        # Human-readable name
     channels: int                    # Max input channels
     sample_rate: int                 # Default sample rate
-    backend: str                     # Backend name (pulseaudio, pipewire, portaudio)
+    backend: str                     # Backend name ('alsa')
     is_default: bool = False         # Is system default
     is_monitor: bool = False         # Is a monitor/loopback source
     description: str = ""            # Extended description