Add token counts, timestamps, and model to rollouts

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This PR adds per-message token count info and timestamps and per-chat selected model info to the rollout JSONL files. Note that this change is only in the Rust version; I didn't add this information to the TypeScript version's JSON rollout files, but I can if desired.

Codex support in Splitrail is now implemented and waiting for this PR.

Related: #1572

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I have read the CLA Document and I hereby sign the CLA

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Ready for review.

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Hi @bolinfest! This PR adds token, timestamp, and model information to rollout files. This enables our new tool, Splitrail, to track token usage, cost, and throughput for Codex users. It also makes it easier for other tools to do the same. Do you mind taking a look? Thank you!

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cc @aibrahim-oai

[aibrahim-oai]

Tools can already calculate tokens used from the rollout info. I don't think there is much benefit in adding them in response items. What is the use?

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Hi @aibrahim-oai! Thank you for reviewing so quickly. Yes, that makes sense, but automatic input caching makes accurate calculations impossible. When input exceeds 1024 tokens, inputs are automatically cached (see here), so there's no way to determine which part of the input was cached and which wasn't; therefore, we can't calculate cost accurately.

In addition, rollouts don't currently store model information, so it's not possible to determine which tokenizer to use, nor can we determine model/token cost for accurate cost calculation. This PR stores model information to address this.

Last but not least, Codex can be used with custom providers, and it would be difficult to perform tokenization on rollouts that use custom models/providers; with open-source models in Ollama, you'd have to download and use a tokenizer, and with providers that don't distribute their tokenizers, you'd have to use an API.