fix(auth): a GitHub account someone has is theirs to switch to where passwords are offered too

Linking refused every GitHub account with `deprecated` where people sign in
with a password, before asking whether the account was someone else's. An
anonymous person tapping Sign in with GitHub, whose app links first and
switches to the account's person when it is taken, so never got to the
switch: the end-to-end test of 21 Oct found it. Nothing is linked either
way; the account someone has is said to be theirs first.

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_base_server/lib/src/auth/github.dart

@@ -208,7 +208,10 @@ Future<String> verifiedGitHubUserId(
 /// nothing changed, when the account belongs to someone else or the person
 /// has another GitHub account, and wherever people sign in with a password
 /// (ADR 0017: GitHub is deprecated, [GitHubSignInRefusal.deprecated])
-/// ([GitHubSignInRefusedException]).
+/// ([GitHubSignInRefusedException]). An account someone else has is said to
+/// be so there too, as nothing would be linked: an anonymous person's app
+/// then switches to it (`gitHubSignIn.signIn`), which still signs in those
+/// who have one.
 Future<void> linkGitHubAccount(
   Session session, {
   required UuidValue authUserId,
@@ -229,16 +232,16 @@ Future<void> linkGitHubAccount(
   )) {
     return;
   }
-  if (passwordSignInAvailable(session.serverpod)) {
-    throw GitHubSignInRefusedException(
-      reason: GitHubSignInRefusal.deprecated,
-    );
-  }
   if (accounts.any((account) => account.userIdentifier == gitHubUserId)) {
     throw GitHubSignInRefusedException(
       reason: GitHubSignInRefusal.takenByAnotherPerson,
     );
   }
+  if (passwordSignInAvailable(session.serverpod)) {
+    throw GitHubSignInRefusedException(
+      reason: GitHubSignInRefusal.deprecated,
+    );
+  }
   if (accounts.isNotEmpty) {
     throw GitHubSignInRefusedException(
       reason: GitHubSignInRefusal.otherGitHubAccountLinked,