feat(oidc): a fake provider for hosts' tests (testing.dart)

The module's test provider moves into `package:halkyon_oidc_server/testing.dart`,
so a host tests its own sign-in against the module's real checks rather than
a copy: `TestKey` makes and publishes an RSA key and signs ID tokens with
headers written by hand (no `typ`, as SimpleLogin's), and `FakeProvider`
answers discovery, the keys and the token endpoint through an HTTP client
the host hands the module. `FakeProvider.simpleLogin` has SimpleLogin's
issuer and endpoints, without PKCE, as the preset expects; a test now signs
in through the preset end to end. pointycastle, already in the lock through
dart_jsonwebtoken, becomes a dependency for it.

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/lib/testing.dart

@@ -0,0 +1,8 @@
+/// A provider for tests, the module's and a host's own: RSA keys made in the
+/// test, and a fake provider answering the module's requests as an OpenID
+/// Connect provider would, SimpleLogin's way where asked. Never for a
+/// server's own code.
+library;
+
+export 'src/testing/fake_provider.dart';
+export 'src/testing/test_key.dart';