fix(ai-gateway-provider): document createUnified structured outputs

[hith3sh]

Summary

• document that createUnified() needs supportsStructuredOutputs: true when routing structured-output-capable upstream models
• add the note directly under the unified dynamic-route example in the README

Why

• issue Schema output with createUnified #540 showed the behavior already worked, but the required option only existed in issue comments
• without the flag, schema output can fall back to plain JSON mode instead of forwarding the full JSON schema upstream

Validation

• ran pnpm exec oxfmt --write packages/ai-gateway-provider/README.md
• no tests run (docs-only change)

Closes #540

[changeset-bot]

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Latest commit: a1d1c0b

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[hith3sh]

Closing this docs-only draft in favor of the code fix in #604.