AI Hybrid Inference: flatten initial prompts type

[erikeldridge]

The Prompt API IDL was recently updated to simplify the way initial prompts are specified:
https://github.com/webmachinelearning/prompt-api/pull/114/files

This change updates the corresponding types used by the hybrid inference feature.

[changeset-bot]

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Latest commit: 5379495

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[github-actions]

Vertex AI Mock Responses Check ⚠️

A newer major version of the mock responses for Vertex AI unit tests is available. update_vertexai_responses.sh should be updated to clone the latest version of the responses: v13.0

[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (177f546) and merge commit (5a615ea).

Test Logs

• Base (177f546): https://github.com/firebase/firebase-js-sdk/actions/runs/15310944302
• Merge (5a615ea): https://github.com/firebase/firebase-js-sdk/actions/runs/15311166180

1. https://storage.googleapis.com/firebase-sdk-metric-reports/PUTxqXssng.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (177f546) and merge commit (5a615ea).

Test Logs

• Base (177f546): https://github.com/firebase/firebase-js-sdk/actions/runs/15310944302
• Merge (5a615ea): https://github.com/firebase/firebase-js-sdk/actions/runs/15311166180

1. https://storage.googleapis.com/firebase-sdk-metric-reports/EMf2z8NB1i.html

[gsiddh]

LGTM. (one outstanding question being discussed in chat)