Preserve OpenAI-shaped Workers AI responses

[mikemikimike]

Fixes #628

Non-streaming Workers AI bindings can return an OpenAI-shaped chat.completion response with a choices array. The existing wrapper assumed the native { response } shape, discarded the content, and caused structured output validation to receive an empty string.

This change preserves OpenAI-shaped responses unchanged and adds a regression test for structured JSON content.

Validation:

• corepack pnpm --filter @cloudflare/tanstack-ai test -- --run test/binding-fetch.test.ts (309 tests passed)
• corepack pnpm --filter @cloudflare/tanstack-ai type-check

[changeset-bot]

🦋 Changeset detected

Latest commit: 59b3636

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@cloudflare/tanstack-ai	Patch

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[mikemikimike]

Validation

Full @cloudflare/tanstack-ai package test suite:

• pnpm --filter @cloudflare/tanstack-ai exec vitest --run
• Result: 18 test files passed, 309 tests passed.
• pnpm changeset status detects the required @cloudflare/tanstack-ai patch changeset.