Avoid duplicate OpenCode Go auth headers

[chrarnoldus]

Summary

• select x-api-key as the OpenCode Go auth header for Messages API requests
• retain bearer authorization for Chat Completions and Responses requests
• remove manual extra-header injection so each request sends exactly one API credential header

Testing

• CI only; isolated dependency setup exceeded the local environment time limit

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (2 files)

• apps/web/src/lib/ai-gateway/providers/direct-byok/opencode-go.ts
• apps/web/src/lib/ai-gateway/providers/direct-byok/opencode-go.test.ts

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_{Reviewed by grok-4.6 · Input: 45.2K · Output: 7.6K · Cached: 314.9K}

_{Review guidance: REVIEW.md from base branch main}