refactor: excerpt as the meta description on the page of post and single page (#3745)

#### What type of PR is this?
/kind improvement
/area core
#### What this PR does / why we need it:
将文章摘要作为 meta description 以优化文章页的 SEO

how to test it?
查看文章页和自定义页面的 head 中是否具有 `<meta name="description" content="文章摘要"/>` 标签

#### Which issue(s) this PR fixes:
Fixes #2682

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

```release-note
将文章摘要作为 meta description 以优化文章页的 SEO
```

application/src/main/java/run/halo/app/theme/dialect/ContentTemplateHeadProcessor.java

@@ -1,5 +1,10 @@
 package run.halo.app.theme.dialect;
 
+import static org.apache.commons.lang3.ObjectUtils.defaultIfNull;
+import static org.apache.commons.lang3.StringUtils.defaultIfBlank;
+
+import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import lombok.AllArgsConstructor;
@@ -37,10 +42,20 @@ public class ContentTemplateHeadProcessor implements TemplateHeadProcessor {
         Mono<List<Map<String, String>>> htmlMetasMono = Mono.empty();
         if (isPostTemplate(context)) {
             htmlMetasMono = nameMono.flatMap(postFinder::getByName)
-                .map(post -> post.getSpec().getHtmlMetas());
+                .map(post -> {
+                    List<Map<String, String>> htmlMetas = post.getSpec().getHtmlMetas();
+                    String excerpt =
+                        post.getStatus() == null ? null : post.getStatus().getExcerpt();
+                    return excerptToMetaDescriptionIfAbsent(htmlMetas, excerpt);
+                });
         } else if (isPageTemplate(context)) {
             htmlMetasMono = nameMono.flatMap(singlePageFinder::getByName)
-                .map(page -> page.getSpec().getHtmlMetas());
+                .map(page -> {
+                    List<Map<String, String>> htmlMetas = page.getSpec().getHtmlMetas();
+                    String excerpt =
+                        page.getStatus() == null ? null : page.getStatus().getExcerpt();
+                    return excerptToMetaDescriptionIfAbsent(htmlMetas, excerpt);
+                });
         }
 
         return htmlMetasMono
@@ -52,6 +67,32 @@ public class ContentTemplateHeadProcessor implements TemplateHeadProcessor {
             .then();
     }
 
+    static List<Map<String, String>> excerptToMetaDescriptionIfAbsent(
+        List<Map<String, String>> htmlMetas,
+        String excerpt) {
+        final String excerptNullSafe = StringUtils.defaultString(excerpt);
+        List<Map<String, String>> metas = new ArrayList<>(defaultIfNull(htmlMetas, List.of()));
+        metas.stream()
+            .filter(map -> Meta.DESCRIPTION.equals(map.get(Meta.NAME)))
+            .distinct()
+            .findFirst()
+            .ifPresentOrElse(map ->
+                    map.put(Meta.CONTENT, defaultIfBlank(map.get(Meta.CONTENT), excerptNullSafe)),
+                () -> {
+                    Map<String, String> map = new HashMap<>();
+                    map.put(Meta.NAME, Meta.DESCRIPTION);
+                    map.put(Meta.CONTENT, excerptNullSafe);
+                    metas.add(map);
+                });
+        return metas;
+    }
+
+    interface Meta {
+        String DESCRIPTION = "description";
+        String NAME = "name";
+        String CONTENT = "content";
+    }
+
     private String headMetaBuilder(List<Map<String, String>> htmlMetas) {
         if (htmlMetas == null) {
             return StringUtils.EMPTY;

application/src/test/java/run/halo/app/theme/dialect/ContentTemplateHeadProcessorTest.java

@@ -0,0 +1,88 @@
+package run.halo.app.theme.dialect;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests for {@link ContentTemplateHeadProcessor}.
+ *
+ * @author guqing
+ * @since 2.5.0
+ */
+class ContentTemplateHeadProcessorTest {
+
+    @Nested
+    class ExcerptToMetaDescriptionTest {
+        @Test
+        void toMetaWhenExcerptIsNull() {
+            List<Map<String, String>> htmlMetas = new ArrayList<>();
+            htmlMetas.add(createMetaMap("keywords", "test"));
+            var result = ContentTemplateHeadProcessor.excerptToMetaDescriptionIfAbsent(htmlMetas,
+                null);
+            assertThat(result).hasSize(2);
+            assertThat(result.get(0)).containsEntry("name", "keywords");
+            assertThat(result.get(1)).containsEntry("name", "description")
+                .containsEntry("content", "");
+        }
+
+        @Test
+        void toMetaWhenWhenHtmlMetaIsNull() {
+            var result = ContentTemplateHeadProcessor.excerptToMetaDescriptionIfAbsent(null,
+                null);
+            assertThat(result).hasSize(1);
+            assertThat(result.get(0)).containsEntry("name", "description")
+                .containsEntry("content", "");
+        }
+
+        @Test
+        void toMetaWhenWhenExcerptNotEmpty() {
+            List<Map<String, String>> htmlMetas = new ArrayList<>();
+            htmlMetas.add(createMetaMap("keywords", "test"));
+            var result = ContentTemplateHeadProcessor.excerptToMetaDescriptionIfAbsent(htmlMetas,
+                "test excerpt");
+            assertThat(result).hasSize(2);
+            assertThat(result.get(0)).containsEntry("name", "keywords");
+            assertThat(result.get(1)).containsEntry("name", "description")
+                .containsEntry("content", "test excerpt");
+        }
+
+        @Test
+        void toMetaWhenWhenDescriptionExistsAndEmpty() {
+            List<Map<String, String>> htmlMetas = new ArrayList<>();
+            htmlMetas.add(createMetaMap("keywords", "test"));
+            htmlMetas.add(createMetaMap("description", ""));
+            var result = ContentTemplateHeadProcessor.excerptToMetaDescriptionIfAbsent(htmlMetas,
+                "test excerpt");
+            assertThat(result).hasSize(2);
+            assertThat(result.get(0)).containsEntry("name", "keywords");
+            assertThat(result.get(1)).containsEntry("name", "description")
+                .containsEntry("content", "test excerpt");
+        }
+
+        @Test
+        void toMetaWhenWhenDescriptionExistsAndNotEmpty() {
+            List<Map<String, String>> htmlMetas = new ArrayList<>();
+            htmlMetas.add(createMetaMap("keywords", "test"));
+            htmlMetas.add(createMetaMap("description", "test description"));
+            var result = ContentTemplateHeadProcessor.excerptToMetaDescriptionIfAbsent(htmlMetas,
+                "test excerpt");
+            assertThat(result).hasSize(2);
+            assertThat(result.get(0)).containsEntry("name", "keywords");
+            assertThat(result.get(1)).containsEntry("name", "description")
+                .containsEntry("content", "test description");
+        }
+
+        Map<String, String> createMetaMap(String nameValue, String contentValue) {
+            Map<String, String> metaMap = new HashMap<>();
+            metaMap.put("name", nameValue);
+            metaMap.put("content", contentValue);
+            return metaMap;
+        }
+    }
+}

application/src/test/java/run/halo/app/theme/dialect/HaloProcessorDialectTest.java

@@ -169,6 +169,7 @@ class HaloProcessorDialectTest {
                 <title>Post</title>
               <meta content="post-meta-V1" name="post-meta-V1" />
             <meta content="post-meta-V2" name="post-meta-V2" />
+            <meta name="description" content="" />
             <meta name="global-head-test" content="test" />
             <meta name="content-head-test" content="test" />
             </head>