test(opencode): retain endpoint-specific auth for paid JSON requests

[diegosouzapw]

Summary

Preserve the non-streaming header contract while asserting protocol-specific authentication: paid OpenCode Zen Responses requests use x-api-key; Chat Completions requests use Bearer. Both omit Accept and forward the checked headers to the HTTP boundary.

This retains the intentional #13247 / #12633 production behavior. No production code, timeout or auth-policy changes.

Evidence

• Exact base 373c31f after checkout completion: stale Bearer assertion fails for the Responses model.
• Updated executor suite plus original fix(providers): OpenCode Zen Responses models reject OmniRoute auth header #12633 auth regressions: 63 PASS, zero failures/skips.
• Prettier, ESLint with frozen existing suppressions, diff check and full branch hooks passed.
• An earlier attempt overlapped worktree materialization and failed module resolution; it is a setup error, not the RED proof above.

Draft recovery slice. Full combined CI and rollout remain pending.

[diegosouzapw]

Fechando como coberta pela #14511, mergeada na release/v3.8.51 (drenou as mesmas base-reds: genérico de stdio do auggie, imports do projectCombo, strip de _native*Passthrough e os guards de catálogo/prefixo/qwen/opencode/quota). O conteúdo que só esta PR tinha já entrou pelas #14501, #14503, #14508 e #14516. (merge-batch 2026-09-24)