feat(ai-gateway): add TypeSafe connection provider

[yardend-wix]

Summary

Allow aiGateway.connection({ provider: 'typesafe' }) to return the TypeSafe-compatible gateway base URL. Existing connection() calls keep the same OpenAI URL and authentication behavior.

Intent (for reviewers & PR Autofix)

What this PR does

Add and export an optional provider selector for the existing synchronous connection helper, shared by user and service-role clients.

Key decisions & why

• provider selects the compatible endpoint contract; model selection remains with the external client adapter.
• A closed runtime allowlist rejects unsupported values instead of inserting arbitrary values into a URL.
• Preserve current token selection and backend-request host propagation without new dependencies or network calls.

What NOT to touch

The default OpenAI behavior and existing client factories are intentionally unchanged. This helper does not hold provider credentials or implement evaluation itself.

Tradeoffs / follow-ups

TypeSafe calls require the separately implemented backend endpoint. The builder guide must pin a release containing this change before exposure; this PR does not publish a stable SDK release.

Testing

• 303 unit tests passed, including provider selection, invalid values, both token modes, and backend-request host propagation.
• Public type tests, build, lint, generated-reference checks, and diff checks passed.
• No dependency, lockfile, or package-version changes.

[github-actions]

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