Restore TestRestTemplate#getRootUri

This commit adapts TestRestTemplate#getRootUri to the recently
introduced LocalTestWebServer. The behavior invokes the
UriTemplateHandler to provide the value. If the implementation expands
full URL, then it will provide the expected behavior. If not, it should
return the empty string as before.

Closes gh-48330

module/spring-boot-resttestclient/src/main/java/org/springframework/boot/resttestclient/TestRestTemplate.java

@@ -193,7 +193,7 @@ public class TestRestTemplate {
 		if (uriTemplateHandler instanceof RootUriTemplateHandler rootHandler) {
 			return rootHandler.getRootUri();
 		}
-		return "";
+		return uriTemplateHandler.expand("").toString();
 	}
 
 	/**

module/spring-boot-resttestclient/src/test/java/org/springframework/boot/resttestclient/TestRestTemplateTests.java

@@ -120,6 +120,14 @@ class TestRestTemplateTests {
 		assertThat(new TestRestTemplate(delegate).getRootUri()).isEqualTo(rootUri);
 	}
 
+	@Test
+	void getRootUriRootUriSetViaLocalTestWebServer() {
+		LocalTestWebServer localTestWebServer = LocalTestWebServer.of(Scheme.HTTPS, 7070);
+		RestTemplateBuilder delegate = new RestTemplateBuilder()
+			.uriTemplateHandler(localTestWebServer.uriBuilderFactory());
+		assertThat(new TestRestTemplate(delegate).getRootUri()).isEqualTo("https://localhost:7070");
+	}
+
 	@Test
 	void getRootUriRootUriNotSet() {
 		assertThat(new TestRestTemplate().getRootUri()).isEmpty();