fix(cost): account for OpenRouter cache write tokens

[LucasSantana-Dev]

Issue for this PR

Closes #18440

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This fixes OpenRouter cost underestimation when cache write tokens are present. OpenAI-compatible responses can report prompt_tokens_details.cache_write_tokens, but we were not carrying that through for usage accounting.

Changes in this PR:

• parse cache_write_tokens in OpenAI-compatible provider paths (stream + non-stream)
• persist that value into provider metadata (openrouter.usage.cacheWriteInputTokens)
• include that metadata fallback in Session.getUsage so cache write tokens are counted in token/cost totals
• add regression coverage for the OpenRouter metadata path in Session.getUsage tests

How did you verify your code works?

• cd packages/opencode && bun test test/session/compaction.test.ts
• cd packages/opencode && bun run typecheck

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[rekram1-node]

I'm having a hard time trusting these PRs because if u look at the file paths u edited, it specifically says it is specifically and ONLY for copilot, so what does that have to do with openrouter?

That codepath should basically never be hit unless u have a custom provider configured to use the github copilot sdk which I kinda doubt?

[rekram1-node]

It even has a readme saying as much:
https://github.com/anomalyco/opencode/tree/dev/packages/opencode/src/provider/sdk/copilot#readme

[LucasSantana-Dev]

It even has a readme saying as much:

https://github.com/anomalyco/opencode/tree/dev/packages/opencode/src/provider/sdk/copilot#readme

I'll have a second look on that and figure out a more structured and fitting solution for the idea i had!