feat(firebaseai): bidi transcript

[cynthiajoan]

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[cynthiajoan]

/gemini summarize

[gemini-code-assist]

This pull request introduces bidirectional transcription capabilities for Firebase AI. Key changes include:

• New Configuration: Adds AudioTranscriptionConfig and integrates inputAudioTranscription and outputAudioTranscription into LiveGenerationConfig to enable transcription for both input and output audio streams.
• Transcription Data Model: Introduces a Transcription class to represent transcription text and its completion status.
• Live Server Content: Extends LiveServerContent to include inputTranscription and outputTranscription fields, allowing the live server to send transcription updates.
• Example App Updates: The example application (bidi_page.dart) has been updated to display these new transcription messages in real-time, including logic to append new transcription segments to existing messages. The text field in MessageData was made mutable to facilitate these updates.

This feature enhances the live generation experience by providing real-time text representations of both user input and model output audio.