docs: describe what to do when test fails

This test is quite important, because it ensure we don't forget
supporting RPCs in ZPT. To help clarify this, I've added instructions in
case the test fails.

(cherry picked from commit 46a2e5c)

engine/src/test/java/io/camunda/zeebe/process/test/engine/GrpcToLogStreamGatewayTest.java

@@ -44,7 +44,13 @@ void testImplementsGatewayEndpoint(final String methodName) {
             .findAny();
 
     assertThat(optionalMethod)
-        .describedAs("Expected method %s to be implemented", methodName)
+        .describedAs(
+            """
+            Expected method %s to be implemented. \
+            When this test fails, it's likely a new RPC that ZPT should support. \
+            Please check whether it should be supported by ZPT. \
+            If it should be suported, add a test case to EngineClientTest.java""",
+            methodName)
         .isPresent();
   }