fix(provider): scope AI Gateway token to first-party Workers AI models

[keefetang]

Issue for this PR

Closes #32051
Closes #40344

Reapplies and extends #33407 (auto-closed on a PR-template technicality).
Fixes the third-party regression introduced by #32052.

Type of change

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

What does this PR do?

#32052 fixed #32051 (Workers AI models 401'd through the gateway) by passing
apiKey to createUnified. That apiKey was applied to every model, so the
Cloudflare API token was sent as the upstream Authorization header for
third-party providers (OpenAI, Anthropic) too — causing them to 401 with
"Invalid API Key".

This scopes token forwarding to be model-aware. The Cloudflare token is only
attached for first-party Workers AI models, whose upstream is Cloudflare
itself; everything else relies on the gateway's stored/BYOK credentials (or
Unified Billing):

const isWorkersAi = modelID.startsWith("workers-ai/") || modelID.startsWith("@cf/")
createUnified(isWorkersAi ? { apiKey } : {})

Both Workers AI id forms the Unified API accepts are matched: the explicit
workers-ai/... prefix and the bare @cf/... form. @cf/ is Cloudflare's
reserved namespace, so this never matches a third-party model.

Applied in both the v1 provider path (provider.ts) and the v2 plugin path
(core/.../cloudflare-ai-gateway.ts). The gateway-level auth
(createAiGateway({ apiKey }), the cf-aig-authorization header) is unchanged.

How did you verify your code works?

Added regression tests in packages/opencode/test/provider/cf-ai-gateway-e2e.test.ts
that capture the real forwarded headers and assert both directions:

• third-party models do not forward the Cloudflare token upstream — the
  upstream sub-request carries no authorization header (fix(provider): pass apiKey to createUnified for Cloudflare AI Gateway #32052 regression)
• workers-ai/... and bare @cf/... models do forward it (cloudflare-ai-gateway provider returns 401 for Workers AI models #32051)

cd packages/opencode
bun test test/provider/cf-ai-gateway-e2e.test.ts   # 6 pass
bun typecheck                                       # clean

Screenshots / recordings

N/A — no UI changes.

Checklist

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

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[rekram1-node]

this fell under the radar will go over this today