feat: add tailor.aigateway.get types for AI Gateway URL resolution

[dragon3]

Summary

Adds type declarations for the tailor.aigateway.get(name) built-in, which resolves a named AI Gateway in the function's own workspace and returns its URL (verified to exist at call time).

declare namespace tailor.aigateway {
  interface AIGateway {
    url: string;
  }
  function get(name: string): Promise<AIGateway>;
}

Usage:

const gw = await tailor.aigateway.get("my-aigateway");
const res = await fetch(`${gw.url}/v1/chat/completions`, { method: "POST", /* ... */ });

The return value is an AIGateway object (not a bare string) so future fields can be added without breaking callers. Mirrors the tailor.<namespace>.<method> shape used by tailor.secretmanager / tailor.idp.

A changeset is included (minor bump for @tailor-platform/function-types).

[k1LoW]

LGTM!

[dragon3]

Thank you for reviewing 🙇‍♂️ I'm going to merge it 💪