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Contribution guide.

Is your feature request related to a problem? Please describe.
Currently, ADK live enable audio transcription only for multi-agent scenarios (https://github.com/google/adk-python/blob/main/src/google/adk/runners.py#L939-L971). For single agent voice use case, the audio transcription will provide a persistent record, which is useful for debugging or history search.

Describe the solution you'd like
Enable transcription for single-agent live scenarios, not just multi-agent live scenarios.

Describe alternatives you've considered
Users can manually config.

Additional context
I will raise a PR once gather feedbacks for this feature request.