refactor: remove trailing slash in site url for notification (#6660)

#### What type of PR is this?
/kind improvement
/area core
/milestone 2.20.x

#### What this PR does / why we need it:
修复 external-url 配置带了尾部斜杠导致邮件通知的查看通知链接无法访问的问题

#### Which issue(s) this PR fixes:
Fixes #6655

#### Does this PR introduce a user-facing change?
```release-note
修复 external-url 配置带了尾部斜杠导致邮件通知的查看通知链接无法访问的问题
```

application/src/main/java/run/halo/app/notification/DefaultNotificationTemplateRender.java

@@ -5,7 +5,9 @@ import static org.apache.commons.lang3.StringUtils.defaultString;
 import java.util.HashMap;
 import java.util.Locale;
 import java.util.Map;
+import java.util.Optional;
 import lombok.RequiredArgsConstructor;
+import org.apache.commons.lang3.StringUtils;
 import org.springframework.stereotype.Component;
 import org.thymeleaf.TemplateEngine;
 import org.thymeleaf.context.Context;
@@ -40,13 +42,16 @@ public class DefaultNotificationTemplateRender implements NotificationTemplateRe
     @Override
     public Mono<String> render(String template, Map<String, Object> model) {
         var context = new Context(Locale.getDefault(), model);
+        var externalUrl = Optional.ofNullable(externalUrlSupplier.getRaw())
+            .map(url -> StringUtils.removeEnd(url.toString(), "/"))
+            .orElse(StringUtils.EMPTY);
         var globalAttributeMono = getBasicSetting()
             .doOnNext(basic -> {
                 var site = new HashMap<>();
                 site.put("title", basic.getTitle());
                 site.put("logo", basic.getLogo());
                 site.put("subtitle", basic.getSubtitle());
-                site.put("url", externalUrlSupplier.getRaw());
+                site.put("url", externalUrl);
                 context.setVariable("site", site);
             });
         return Mono.when(globalAttributeMono)

application/src/test/java/run/halo/app/notification/DefaultNotificationTemplateRenderTest.java

@@ -9,6 +9,7 @@ import static org.mockito.Mockito.when;
 
 import java.net.MalformedURLException;
 import java.net.URI;
+import java.net.URL;
 import java.util.Map;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -88,7 +89,7 @@ class DefaultNotificationTemplateRenderTest {
                       以下是回复的具体内容：
                       
                       这是回复的内容
-   
+                       
                     Halo
                     http://localhost:8090
                     祝好！
@@ -101,4 +102,27 @@ class DefaultNotificationTemplateRenderTest {
             eq(SystemSetting.Basic.class));
         verify(externalUrlSupplier).getRaw();
     }
-}
+
+    @Test
+    void siteUrlTest() throws MalformedURLException {
+        when(environmentFetcher.fetch(eq(SystemSetting.Basic.GROUP), eq(SystemSetting.Basic.class)))
+            .thenReturn(Mono.just(new SystemSetting.Basic()));
+
+        var template = "<a th:href=\"|${site.url}/uc/notifications|\">查看通知</a>";
+        var expected = "<a href=\"http://localhost:8090/uc/notifications\">查看通知</a>";
+
+        when(externalUrlSupplier.getRaw()).thenReturn(new URL("http://localhost:8090/"));
+        templateRender.render(template,
+                Map.of())
+            .as(StepVerifier::create)
+            .expectNext(expected)
+            .verifyComplete();
+
+        when(externalUrlSupplier.getRaw()).thenReturn(new URL("http://localhost:8090"));
+        templateRender.render(template,
+                Map.of())
+            .as(StepVerifier::create)
+            .expectNext(expected)
+            .verifyComplete();
+    }
+}