feat: Add inputAudioTranscription support to Java ADK

[jinnigu]

Summary

Adds inputAudioTranscription support to the Java ADK to achieve feature parity with Python. When enabled, the live connect config requests model-side transcription of input audio into text, allowing real-time processing of spoken input in live streaming scenarios.

Changes

Core Implementation

• RunConfig: Added inputAudioTranscription field with getter/setter and builder support
• Basic: Maps RunConfig.inputAudioTranscription to LiveConnectConfig.inputTranscription for model-side transcription
• Runner: Auto-enables input/output transcription for live multi-agent scenarios to match Python behavior

Bug Fix

• Runner: Fixed unreachable condition in newInvocationContextForLive() where the outer check !CollectionUtils.isNullOrEmpty(runConfig.responseModalities()) (NOT empty) made the inner check CollectionUtils.isNullOrEmpty(runConfig.responseModalities()) (IS empty) impossible to reach. This prevented the "default to AUDIO modality" logic from ever executing.

Behavior Alignment with Python

• Auto-sets inputAudioTranscription only for live multi-agent runs (when agent.subAgents() is non-empty)
• Auto-sets outputAudioTranscription when response modalities imply audio usage
• Leaves transcription settings unchanged for single-agent scenarios

Testing

• Added unit tests for RunConfig transcription field handling
• Added unit tests for Basic flow mapping to LiveConnectConfig

[vorburger]

@jinnigu Thank You for contributing this! Is there any way to illustrate that this actually really 😆 fully works, in this PR? The unit tests are... well, unit tests. Would a full-blown integration test for this be possible? Or, how would you feel about if I invite you to add, as part of this PR, a very (most) simple "MVP" in tutorials/audio with just a super simple LlmAgent (without even any sub-agents), with just an AdkWebServer.start(), which allows us to "see this work in action"? That would be awesome!

[vorburger]

@jinnigu The inline comment and the code don't seem to align, here? The "text" says "apply modality defaults" but then this removes !CollectionUtils.isNullOrEmpty(runConfig.responseModalities()... is that intentional? (It may well be, I'm entirely sure about why this was originally like this; but it seems worth double checking.) Also, why would we limit transcription only for multi-agent live scenarios? I would personally love to use this even for a very simple trivial only-LlmAgent use case... you speak to it, and get a persistent transcript in your session store, that's very cool! I'd love to use this e.g. in my (personal) https://docs.enola.dev project - but don't see why it needs to be limited to work only if !this.agent.subAgents().isEmpty().

[vorburger]

/gemini review

[gemini-code-assist]

Code Review

This pull request successfully adds inputAudioTranscription support to the Java ADK, achieving feature parity with the Python version. The changes are well-structured, including updates to RunConfig, the Basic flow, and the Runner. A significant improvement is the fix for an unreachable code block in Runner.java, which enhances correctness. The new functionality is also thoroughly covered by unit tests. I have one suggestion to refactor a small portion of the logic in Runner.java to improve maintainability by removing duplicated code. Overall, this is a solid and valuable contribution.