Token set check: drop the array clause the access_token check already decides

A JSON array has no string access_token, so the typeof check refuses it;
the separate Array.isArray clause was an equivalent mutant's worth of code.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/oauth.ts

@@ -188,7 +188,6 @@ export async function requestProviderTokens(
   if (
     typeof answer !== "object" ||
     answer === null ||
-    Array.isArray(answer) ||
     typeof (answer as { access_token?: unknown }).access_token !== "string" ||
     !(answer as { access_token: string }).access_token ||
     (answer as { error?: unknown }).error !== undefined