Prevent private posts from being indexed when rebuilding index. (#4882)

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#### What type of PR is this?
/kind bug
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#### What this PR does / why we need it:

在加入索引前判断文章的可见性。只将PUBLIC文章加入索引，修复手动刷新搜索索引会将私有文章加入索引的问题

#### Which issue(s) this PR fixes:
Fix #4879 

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#### Special notes for your reviewer:

null

#### Does this PR introduce a user-facing change?

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```release-note
修复手动刷新搜索索引会将私有文章加入索引的问题
```

api/src/main/java/run/halo/app/core/extension/content/Post.java

@@ -81,6 +81,10 @@ public class Post extends AbstractExtension {
         return labels != null && parseBoolean(labels.getOrDefault(PUBLISHED_LABEL, "false"));
     }
 
+    public static boolean isPublic(PostSpec spec) {
+        return spec.getVisible() == null || VisibleEnum.PUBLIC.equals(spec.getVisible());
+    }
+
     @Data
     public static class PostSpec {
         @Schema(requiredMode = RequiredMode.REQUIRED, minLength = 1)

application/src/main/java/run/halo/app/search/IndicesServiceImpl.java

@@ -40,7 +40,8 @@ public class IndicesServiceImpl implements IndicesService {
 
     private Mono<Void> rebuildPostIndices(PostSearchService searchService) {
         return postFinder.listAll()
-            .filter(post -> Post.isPublished(post.getMetadata()))
+            .filter(
+                post -> Post.isPublished(post.getMetadata()) && Post.isPublic(post.getSpec()))
             .flatMap(listedPostVo -> {
                 PostVo postVo = PostVo.from(listedPostVo);
                 return postFinder.content(postVo.getMetadata().getName())