fix(oauth): declare searchParams in register-session POST handler

Missing declaration caused a ReferenceError -> 500 HTML response instead
of JSON when clients called POST .../register-session.

src/app/api/oauth/[provider]/[action]/route.js

@@ -254,6 +254,7 @@ export async function POST(request, { params }) {
     if (action === "register-session") {
       // Register proxy session out of URL query (state) + body (codeVerifier).
       // Zed's codeVerifier encodes the RSA private key — must stay out of URL/logs.
+      const searchParams = new URL(request.url).searchParams;
       const state = searchParams.get("state") || body?.state;
       if (!state) return NextResponse.json({ error: "Missing state" }, { status: 400 });
       let ok = false;