fix(ai): skip zen tests when OPENCODE_API_KEY is not set (#605)

The zen.test.ts file was added without the standard skipIf guard,
causing CI to fail on all PRs since no OPENCODE_API_KEY is configured.

This follows the same pattern used by other API-dependent tests.

packages/ai/test/zen.test.ts

@@ -3,7 +3,7 @@ import { MODELS } from "../src/models.generated.js";
 import { complete } from "../src/stream.js";
 import type { Model } from "../src/types.js";
 
-describe("OpenCode Zen Models Smoke Test", () => {
+describe.skipIf(!process.env.OPENCODE_API_KEY)("OpenCode Zen Models Smoke Test", () => {
 	const zenModels = Object.values(MODELS.opencode);
 
 	zenModels.forEach((model) => {