Fix handling of additional-hosts with Mongo

Closes gh-36441

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/mongo/PropertiesMongoConnectionDetails.java

@@ -61,6 +61,7 @@ public class PropertiesMongoConnectionDetails implements MongoConnectionDetails
 			builder.append(this.properties.getPort());
 		}
 		if (this.properties.getAdditionalHosts() != null) {
+			builder.append(",");
 			builder.append(String.join(",", this.properties.getAdditionalHosts()));
 		}
 		builder.append("/");

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/mongo/PropertiesMongoConnectionDetailsTests.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.autoconfigure.mongo;
 
+import java.util.List;
+
 import com.mongodb.ConnectionString;
 import org.junit.jupiter.api.Test;
 
@@ -102,6 +104,15 @@ class PropertiesMongoConnectionDetailsTests {
 		assertThat(connectionString.getRequiredReplicaSetName()).isEqualTo("test");
 	}
 
+	@Test
+	void whenAdditionalHostsAreConfiguredThenTheyAreIncludedInHostsOfConnectionString() {
+		this.properties.setHost("mongo1.example.com");
+		this.properties.setAdditionalHosts(List.of("mongo2.example.com", "mongo3.example.com"));
+		ConnectionString connectionString = getConnectionString();
+		assertThat(connectionString.getHosts()).containsExactly("mongo1.example.com", "mongo2.example.com",
+				"mongo3.example.com");
+	}
+
 	private PropertiesMongoConnectionDetails createConnectionDetails() {
 		return new PropertiesMongoConnectionDetails(this.properties);
 	}