feat: return usage for openai providers

[tingwai]

Description

[ What changed? Feel free to be brief. ]

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• [] I've read the contributing guide
• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created

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Tests

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Summary by cubic

Adds token usage reporting (prompt, completion, total) to Gemini and Relace adapters for both streaming and non‑stream responses, enabling cost and quota tracking. Tests updated to expect usage for supported providers.

• New Features

  • Gemini: collect usageMetadata during stream and emit a final usageChatChunk; include usage in non‑stream ChatCompletion.
  • Relace: emit a final usageChatChunk after apply; include usage in non‑stream ChatCompletion.
  • Safer stream parsing when choices may be empty.

• Migration

  • No breaking changes. usage is optional but now populated when available in both streaming chunks and non‑stream results.

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[tingwai]

PR not ready yet, putting it up to run tests

[chezsmithy]

Is this planned to have a plugin to the UI at some point in time, or is it a hook to backend processes. More curious than anything...

[Patrick-Erichsen]

Thanks for updating the tests! Lmk your thoughts on UsageInfo.

[Patrick-Erichsen]

I think both of the UsageInfo types in this PR are actually from CompletionUsage in openai/resources/index ? If we just need a subset of the properties we could use an Omit or Pick.

[tingwai]

Is this planned to have a plugin to the UI at some point in time, or is it a hook to backend processes. More curious than anything...

@chezsmithy This is going to be used for our updated billing system, coming very soon!

[sestinj]

🎉 This PR is included in version 1.13.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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