test(audio): Add error handling tests for WavDumpBackend

Added comprehensive error handling tests to verify WavDumpBackend
handles invalid file paths gracefully without crashes.

New test: test_invalid_file_paths()
- Tests null filename (nullptr)
- Tests non-existent directory path
- Tests permission denied (root directory write)

All cases verify:
- Error message is printed to stderr
- No crash or abort()
- write_audio() does nothing (no segfault)
- samples_written counter stays at 0
- shutdown() handles nullptr gracefully

Example output:
  Error: Failed to open WAV file: (null)
  ✓ Null filename handled gracefully

  Error: Failed to open WAV file: /nonexistent/directory/test.wav
  ✓ Invalid directory path handled gracefully

  Error: Failed to open WAV file: /test.wav
  ✓ Permission denied handled gracefully

This improves test coverage by verifying error paths that could
cause crashes or undefined behavior in production.

All 27 tests pass (including new error handling tests).

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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