Fix CommonPrefixes rendering in S3HttpHandler (elastic#126147)

Today the `ListObjects` implementation in `S3HttpHandler` will put all
the common prefixes in a single `<CommonPrefixes>` container, but in
fact the real S3 gives each one its own container. The v1 SDK is lenient
and accepts either, but the v2 SDK requires us to do this correctly.
This commit fixes the test fixture to match the behaviour of the real
S3.

Author: DaveCTurner

test/fixtures/s3-fixture/src/main/java/fixture/s3/S3HttpHandler.java

@@ -240,12 +240,9 @@ public void handle(final HttpExchange exchange) throws IOException {
                     list.append("<Size>").append(blob.getValue().length()).append("</Size>");
                     list.append("</Contents>");
                 }
-                if (commonPrefixes.isEmpty() == false) {
-                    list.append("<CommonPrefixes>");
-                    commonPrefixes.forEach(commonPrefix -> list.append("<Prefix>").append(commonPrefix).append("</Prefix>"));
-                    list.append("</CommonPrefixes>");
-
-                }
+                commonPrefixes.forEach(
+                    commonPrefix -> list.append("<CommonPrefixes><Prefix>").append(commonPrefix).append("</Prefix></CommonPrefixes>")
+                );
                 list.append("</ListBucketResult>");
 
                 byte[] response = list.toString().getBytes(StandardCharsets.UTF_8);

test/fixtures/s3-fixture/src/test/java/fixture/s3/S3HttpHandlerTests.java

@@ -76,9 +76,27 @@ public void testSimpleObjectOperations() {
             handleRequest(handler, "GET", "/bucket/?prefix=path/other")
         );
 
+        assertEquals(RestStatus.OK, handleRequest(handler, "PUT", "/bucket/path/subpath1/blob", randomAlphaOfLength(50)).status());
+        assertEquals(RestStatus.OK, handleRequest(handler, "PUT", "/bucket/path/subpath2/blob", randomAlphaOfLength(50)).status());
+        assertEquals(new TestHttpResponse(RestStatus.OK, """
+            <?xml version="1.0" encoding="UTF-8"?><ListBucketResult><Prefix>path/</Prefix>\
+            <Delimiter>/</Delimiter><Contents><Key>path/blob</Key><Size>50</Size></Contents>\
+            <CommonPrefixes><Prefix>path/subpath1/</Prefix></CommonPrefixes>\
+            <CommonPrefixes><Prefix>path/subpath2/</Prefix></CommonPrefixes>\
+            </ListBucketResult>"""), handleRequest(handler, "GET", "/bucket/?delimiter=/&prefix=path/"));
+
         assertEquals(RestStatus.OK, handleRequest(handler, "DELETE", "/bucket/path/blob").status());
         assertEquals(RestStatus.NO_CONTENT, handleRequest(handler, "DELETE", "/bucket/path/blob").status());
 
+        assertEquals(new TestHttpResponse(RestStatus.OK, """
+            <?xml version="1.0" encoding="UTF-8"?><ListBucketResult><Prefix></Prefix>\
+            <Contents><Key>path/subpath1/blob</Key><Size>50</Size></Contents>\
+            <Contents><Key>path/subpath2/blob</Key><Size>50</Size></Contents>\
+            </ListBucketResult>"""), handleRequest(handler, "GET", "/bucket/?prefix="));
+
+        assertEquals(RestStatus.OK, handleRequest(handler, "DELETE", "/bucket/path/subpath1/blob").status());
+        assertEquals(RestStatus.OK, handleRequest(handler, "DELETE", "/bucket/path/subpath2/blob").status());
+
         assertEquals(
             new TestHttpResponse(RestStatus.OK, """
                 <?xml version="1.0" encoding="UTF-8"?><ListBucketResult><Prefix></Prefix></ListBucketResult>"""),