feat: add transcription fields mapping

[zigzago]

Problem:
I implemented the Python bidi demo in Kotlin. The behavior matches the Python version, except for the STT transcriptions, which are currently not exposed by the Java ADK SDK.

Solution:

Add the transcription fields to the mapping.

Testing Plan

Tested with the bidi demo. I may contribute this demo to the ADK samples project.

Unit Tests:

• I have added or updated unit tests for my change. (generated using AI - they look fine)
• All unit tests pass locally.

[WARNING] Tests run: 1093, Failures: 0, Errors: 0, Skipped: 10

Manual End-to-End (E2E) Tests:

Tested using the bidi demo

Checklist

• I have read the CONTRIBUTING.md document.
• My pull request contains a single commit.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.