Decode the login-state envelope in the auth-session CSRF scenario

The state parameter is now base64url(JSON { nonce, returnTo }), so the
scenario decodes it and asserts the nonce inside instead of expecting
the raw hex value.

Author: RhysSullivan

e2e/cloud/auth-session.test.ts

@@ -5,7 +5,7 @@
 // callback refusing forged/incomplete redirects, the sealed-session cookie
 // actually authorizing the session API, and logout dropping the cookie.
 import { expect } from "@effect/vitest";
-import { Effect } from "effect";
+import { Effect, Encoding, Result, Schema } from "effect";
 
 import { scenario } from "../src/scenario";
 import { Api, Target } from "../src/services";
@@ -33,7 +33,18 @@ scenario(
 
     const authorizeUrl = new URL(response.headers.get("location") ?? "");
     const state = authorizeUrl.searchParams.get("state") ?? "";
-    expect(state, "the redirect carries an unguessable CSRF state").toMatch(/^[0-9a-f]{64}$/);
+    // state = base64url(JSON { nonce, returnTo? }) — the nonce is the CSRF
+    // secret; returnTo (absent here, no query was passed) rides beside it.
+    const decoded = Schema.decodeUnknownOption(
+      Schema.fromJsonString(
+        Schema.Struct({ nonce: Schema.String, returnTo: Schema.optional(Schema.String) }),
+      ),
+    )(Result.getOrElse(Encoding.decodeBase64UrlString(state), () => ""));
+    expect(decoded._tag, "the state decodes as our login-state envelope").toBe("Some");
+    expect(
+      decoded._tag === "Some" ? decoded.value.nonce : "",
+      "the state carries an unguessable CSRF nonce",
+    ).toMatch(/^[0-9a-f]{64}$/);
     expect(
       authorizeUrl.searchParams.get("redirect_uri"),
       "AuthKit is told to come back to this deployment's callback",