fix(types): restore AudioTranscriptionConfig fields (enable_output, enable_input)

[ubaid2751]

Context

In latest version, the AudioTranscriptionConfig class was reduced to a placeholder,
which broke Gemini Live transcription (no config fields were passed to API).

What this PR does

• Restores enable_output and enable_input fields in AudioTranscriptionConfig
  and AudioTranscriptionConfigDict.
• Adds a test to verify field behavior.

Result

Gemini Live sessions can once again request input/output transcriptions,
consistent with v1.14 behavior.

✅ All existing tests pass.
✅ New test confirms config works.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[ubaid2751]

@janasangeetha Is this going forward ?