fix(ST-38): parse Lyria interaction outputs

[afaryy]

Summary

• Parse the audio and text blocks from the real Lyria Interaction outputs array.
• Preserve the existing safe unavailable fallback for provider failures.
• Add a regression test matching the Google SDK response shape.

Evidence

• Hosted smoke test reached the deployed API with synthetic media and safely returned the fallback.
• Root cause: the previous client expected an output_audio convenience field that the installed SDK does not provide for Interaction responses.
• API suite: 60 passed. One existing Starlette/httpx deprecation warning remains.
• Web suite: 16 passed.
• Production web build: passed.

Follow-up

After merge and automatic deployment, rerun the same non-sensitive hosted export smoke test. Only a successful playable generated-audio ZIP counts as Lyria evidence.