fix(oidc): the fake provider takes the client's credentials form-encoded

The module sends the client's ID and secret form-encoded inside Basic
authentication, as RFC 6749 (2.3.1) has them; the fake compared the header
with them raw, so a host testing with a secret holding a space or a colon
saw every code refused. A test now holds the module to the encoding.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_oidc_server/test/integration/oidc_idp_test.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 
 import 'package:clock/clock.dart';
@@ -699,6 +700,30 @@ void main() {
       expect(fake.discoveries, 0);
     });
 
+    test("sends the client's ID and secret form-encoded, as OAuth 2.0 "
+        'has them in Basic authentication (RFC 6749, 2.3.1)', () async {
+      useProvider(
+        FakeProvider.simpleLogin(
+          key: key,
+          clientId: 'halkyon:learn',
+          clientSecret: 'a secret/+&',
+          callbackUrl: Uri.parse('https://learn.example/simplelogin-callback'),
+        ),
+      );
+      final start = await idp.begin(session, provider: 'simplelogin');
+
+      await idp.verify(
+        session,
+        attempt: start.attempt,
+        code: fake.authorize(start.authorizationUrl, subject: 'sl-1'),
+      );
+
+      expect(
+        fake.tokenRequests.single.headers['authorization'],
+        'Basic ${base64.encode(utf8.encode('halkyon%3Alearn:a+secret%2F%2B%26'))}',
+      );
+    });
+
     test('forgets attempts whose time ran out', () async {
       await idp.begin(session, provider: 'fake');
       final (:attempt, :code) = await allowed('42');