fix(provider): send Anthropic's dashed native slug through the AI Gateway

[superhighfives]

Issue for this PR

Closes #44252

Type of change

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

What does this PR do?

Every Anthropic model under cloudflare-ai-gateway fails with Error: model: claude-haiku-4.5, so none of them are usable.

The plugin already routes anthropic/* models through Anthropic's native Messages passthrough (createAnthropic), which forwards the sliced model id to Anthropic's API unchanged. models.dev lists Cloudflare's canonical dotted ids (anthropic/claude-haiku-4.5), but Anthropic's Messages API expects dashed native slugs (claude-haiku-4-5) — so it 404s.

The fix translates dots to dashes when slicing the anthropic/ prefix. OpenAI is unaffected because its native slugs already use dots (gpt-4.1), which is why only Anthropic was broken.

Worth noting: Cloudflare's own catalog-aware REST endpoints (/ai/v1/messages, /ai/run) accept the dotted id and do this exact dot→dash translation internally before calling Anthropic. The native passthrough route this plugin uses forwards the slug raw, so the plugin has to do the same translation. The dotted id in models.dev is correct — it's Cloudflare's canonical id (see https://developers.cloudflare.com/ai/models/anthropic/claude-haiku-4.5/), so this is fixed on the opencode side, not in the catalog. (Context: surfaced while testing anomalyco/models.dev#4922.)

How did you verify your code works?

• Built the single binary and ran against a live Cloudflare AI Gateway: anthropic/claude-haiku-4.5 went from Error: model: claude-haiku-4.5 to a normal completion; openai/* and workers-ai/@cf/* still work.
• Added a regression test in cf-ai-gateway-e2e.test.ts asserting a dotted Anthropic id reaches the Anthropic Messages wire as a dashed slug; confirmed it fails without the fix and passes with it.
• bun test test/provider/ in packages/opencode passes (one unrelated pre-existing flake in header-timeout.test.ts, which also fails on dev). tsgo --noEmit clean.

Screenshots / recordings

N/A (not a UI change).

Checklist

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

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[superhighfives]

Ah, I think we're using the legacy endpoint here. Closing.