fix: ensure unique platform account binding per user (#63)

### What this PR does?
修复了不同用户无法在同一平台绑定各自账号的问题

/kind bug

Fixes #62 

```release-note
修复了不同用户无法在同一平台绑定各自账号的问题
```

build.gradle

@@ -29,6 +29,6 @@ test {
 }
 
 halo {
-    version = '2.17'
+    version = '2.19'
     debug = true
 }

src/main/java/run/halo/oauth/UserConnectionServiceImpl.java

@@ -40,24 +40,33 @@ public Mono<UserConnection> createConnection(String username,
 
         UserConnection connection = convert(username, authentication);
         String providerUserId = authentication.getPrincipal().getName();
-        return findByRegistrationId(connection.getSpec().getRegistrationId())
-            .hasElement()
-            .flatMap(exists -> {
-                if (exists) {
+        return checkExistingBinding(connection.getSpec().getRegistrationId(), username)
+            .flatMap(existing -> {
+                if (existing) {
                     return Mono.error(new ServerWebInputException(
-                        "已经绑定过 " + connection.getSpec().getRegistrationId() + " 账号，请先解绑"));
+                        "已经绑定过其他 " + connection.getSpec().getRegistrationId()
+                            + " 账号，请先解绑后重试"));
                 }
-                return fetchUserConnection(connection.getSpec().getRegistrationId(), providerUserId)
-                    .flatMap(persisted -> {
-                        connection.getMetadata().setName(persisted.getMetadata().getName());
-                        connection.getMetadata()
-                            .setVersion(persisted.getMetadata().getVersion());
-                        return client.update(connection);
-                    })
-                    .switchIfEmpty(Mono.defer(() -> client.create(connection)));
+                return upsertUserConnection(connection, providerUserId);
             });
     }
 
+    private Mono<Boolean> checkExistingBinding(String registrationId, String username) {
+        return listByRegistrationIdAndUsername(registrationId, username)
+            .hasElements();
+    }
+
+    private Mono<UserConnection> upsertUserConnection(UserConnection connection,
+        String providerUserId) {
+        return fetchUserConnection(connection.getSpec().getRegistrationId(), providerUserId)
+            .flatMap(persisted -> {
+                connection.getMetadata().setName(persisted.getMetadata().getName());
+                connection.getMetadata().setVersion(persisted.getMetadata().getVersion());
+                return client.update(connection);
+            })
+            .switchIfEmpty(Mono.defer(() -> client.create(connection)));
+    }
+
     @Override
     public Flux<UserConnection> removeConnection(String registrationId) {
         return ReactiveSecurityContextHolder.getContext()
@@ -90,12 +99,6 @@ Flux<UserConnection> listByRegistrationIdAndUsername(String registrationId, Stri
             && persisted.getSpec().getUsername().equals(username), null);
     }
 
-    private Mono<UserConnection> findByRegistrationId(String registrationId) {
-        return client.list(UserConnection.class,
-                persisted -> persisted.getSpec().getRegistrationId().equals(registrationId), null)
-            .next();
-    }
-
     private Mono<UserConnection> fetchUserConnection(String registrationId, String providerUserId) {
         return client.list(UserConnection.class, persisted -> persisted.getSpec()
                 .getProviderUserId().equals(providerUserId)