fix: skip Anthropic cache control for OAuth

[jorgitin02]

Issue for this PR

Closes #17910

Type of change

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

What does this PR do?

Anthropic OAuth requests were still going through the normal Anthropic cache-control path, which adds cache_control: { type: "ephemeral" } to recent prompt messages. That metadata is rejected on the OAuth path and causes the request to fail.

This changes ProviderTransform.message() to skip Anthropic cache-control injection when the resolved auth type is oauth, and updates LLM.stream() to pass the resolved auth type into that transform. I also added regression coverage at two levels: one focused transform test for the cache-control behavior, and one LLM.stream() test that exercises the Anthropic messages request path with OAuth auth present.

The fix is narrow on purpose: it only changes Anthropic when auth is OAuth, and leaves the existing non-OAuth Anthropic caching behavior in place.

How did you verify your code works?

• bun test test/session/llm.test.ts
• bun test test/provider/transform.test.ts
• bun typecheck
• bun test

Screenshots / recordings

N/A

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. 👍

[jwcrystal]

⚠️ Update: The cache_control hypothesis has been disproven. See issue comment for details.

The actual root cause is missing x-anthropic-billing-header that Anthropic now requires for OAuth requests. This PR's approach may not fully resolve the issue.

A reference implementation is available at clewdr@a4e5df3.

[jorgitin02]

Updated this PR.

It now does three things for Anthropic OAuth:

• skips injected cache_control
• prepends the Claude Code-style billing header system text
• sends the Claude Code user-agent on OAuth requests

Local verification:

• bun test ./test/provider/transform.test.ts
• bun test ./test/session/llm.test.ts
• bun typecheck

The earlier failing e2e checks were in the app workspace-routing suite, not this provider/session codepath, so those need to be re-evaluated on the refreshed branch checks.