Fix: Ensure consistent audio MIME type

[9chait9]

Fixes #5552

This PR addresses issue #5552 by ensuring that the LiveRequestQueue.send_realtime method consistently sets the audio MIME type. If a types.Blob is passed without an explicit mime_type, it will now default to 'audio/pcm;rate=16000'. Existing mime_type values will be preserved.

Testing Plan

1. Set up a test environment: Configure the ADK with a LiveRequestQueue and a component that sends audio blobs.
2. Scenario 1 (No explicit MIME type): Send an audio blob using LiveRequestQueue.send_realtime without specifying the mime_type in the types.Blob object.
3. Verification 1: Inspect the outgoing request (e.g., using a network proxy or by logging within the LiveRequestQueue implementation) to confirm that the mime_type header or field for the audio blob is automatically set to 'audio/pcm;rate=16000'.
4. Scenario 2 (Explicit MIME type): Send an audio blob using LiveRequestQueue.send_realtime with an explicitly provided mime_type (e.g., 'audio/opus').
5. Verification 2: Inspect the outgoing request to confirm that the explicitly provided mime_type ('audio/opus') is preserved and used.

[adk-bot]

Response from ADK Triaging Agent

Hello @9chait9, thank you for creating this PR!

To help reviewers better understand and test your fix, could you please add a testing plan section to your PR description? Additionally, for bug fixes, providing logs or a screenshot demonstrating the fix would be very helpful.

This information will help reviewers to review your PR more efficiently. Thanks!

[9chait9]

Hi @adk-bot, thank you for the feedback! A detailed testing plan has already been included in the PR description. You can find it under the 'Testing Plan' section. Regarding logs/screenshots, this is an internal fix related to MIME type handling, which isn't easily demonstrated with screenshots. If specific logs are needed, please let me know, and I can provide them.