HLRC: fix uri encode bug when url path starts with '/' (#34436)

This commit sets the authority of a URI to blank such that it does not
misinterpret slashes in the path as the authority.

Closes #34433

client/rest-high-level/src/main/java/org/elasticsearch/client/RequestConverters.java

@@ -1126,7 +1126,9 @@ private static String encodePart(String pathPart) {
                 //encode each part (e.g. index, type and id) separately before merging them into the path
                 //we prepend "/" to the path part to make this path absolute, otherwise there can be issues with
                 //paths that start with `-` or contain `:`
-                URI uri = new URI(null, null, null, -1, "/" + pathPart, null, null);
+                //the authority must be an empty string and not null, else paths that being with slashes could have them
+                //misinterpreted as part of the authority.
+                URI uri = new URI(null, "", "/" + pathPart, null, null);
                 //manually encode any slash that each part may contain
                 return uri.getRawPath().substring(1).replaceAll("/", "%2F");
             } catch (URISyntaxException e) {

client/rest-high-level/src/test/java/org/elasticsearch/client/RequestConvertersTests.java

@@ -1772,6 +1772,38 @@ public void testEndpointBuilderEncodeParts() {
                     .addPathPartAsIs("cache/clear");
             assertEquals("/index1,index2/cache/clear", endpointBuilder.build());
         }
+        {
+            EndpointBuilder endpointBuilder = new EndpointBuilder().addPathPart("/foo");
+            assertEquals("/%2Ffoo", endpointBuilder.build());
+        }
+        {
+            EndpointBuilder endpointBuilder = new EndpointBuilder().addPathPart("//foo");
+            assertEquals("/%2F%2Ffoo", endpointBuilder.build());
+        }
+        {
+            EndpointBuilder endpointBuilder = new EndpointBuilder().addPathPart("///foo");
+            assertEquals("/%2F%2F%2Ffoo", endpointBuilder.build());
+        }
+        {
+            EndpointBuilder endpointBuilder = new EndpointBuilder().addPathPart("/foo/bar");
+            assertEquals("/%2Ffoo%2Fbar", endpointBuilder.build());
+        }
+        {
+            EndpointBuilder endpointBuilder = new EndpointBuilder().addPathPart("//foo/bar");
+            assertEquals("/%2F%2Ffoo%2Fbar", endpointBuilder.build());
+        }
+        {
+            EndpointBuilder endpointBuilder = new EndpointBuilder().addPathPart("/foo@bar");
+            assertEquals("/%2Ffoo@bar", endpointBuilder.build());
+        }
+        {
+            EndpointBuilder endpointBuilder = new EndpointBuilder().addPathPart("//foo@bar");
+            assertEquals("/%2F%2Ffoo@bar", endpointBuilder.build());
+        }
+        {
+            EndpointBuilder endpointBuilder = new EndpointBuilder().addPathPart("/part1").addPathPart("//part2").addPathPart("///part3");
+            assertEquals("/%2Fpart1/%2F%2Fpart2/%2F%2F%2Fpart3", endpointBuilder.build());
+        }
     }
 
     public void testEndpoint() {