fix: clarify Claude Code credential auth error

[Joeyzzyy]

Issue for this PR

Closes #7456

Type of change

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

What does this PR do?

When OpenCode receives Anthropic auth errors for Claude Code-only credentials, the existing error text does not clearly tell the user what to do next.

This change detects that specific credential error and replaces it with clearer guidance that says the credential is restricted to Claude Code, cannot be used with OpenCode, and that the user should use a standard Anthropic API key and run opencode auth login anthropic.

The change is intentionally narrow and only affects provider error messaging for this specific Anthropic auth case.

How did you verify your code works?

• added a focused provider error test for the Claude Code-only credential case
• ran bun test test/provider/error.test.ts
• ran bun typecheck
• verified generic Anthropic auth errors are not rewritten into the Claude Code-specific guidance

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. 👍

[Joeyzzyy]

I checked the failing e2e jobs and they are both failing in packages/app e2e review flows (), which looks unrelated to this provider error messaging change.

The provider-focused test and typecheck pass locally. I have re-run the failed jobs to check whether this is a flaky CI failure.

[Joeyzzyy]

I checked the failing e2e jobs and they are both failing in packages/app e2e review flows (session-review.spec.ts), which looks unrelated to this provider error messaging change.

The provider-focused test and typecheck pass locally. I have re-run the failed jobs to check whether this is a flaky CI failure.