Add assertion to process method (#1453)

Also adjust clangd so it doesn't bother us with header files about `expected`.

src/rpc/handlers/Feature.cpp

@@ -25,6 +25,7 @@
 #include "rpc/common/Specs.hpp"
 #include "rpc/common/Types.hpp"
 #include "rpc/common/Validators.hpp"
+#include "util/Assert.hpp"
 
 #include <boost/json/conversion.hpp>
 #include <boost/json/value.hpp>
@@ -40,6 +41,7 @@ FeatureHandler::process([[maybe_unused]] FeatureHandler::Input input, [[maybe_un
 {
     // For now this handler only fires when "vetoed" is set in the request.
     // This always leads to a `notSupported` error as we don't want anyone to be able to
+    ASSERT(false, "FeatureHandler::process is not implemented.");
     return Output{};
 }