refactor: Enable clang-tidy bugprone-unused-return-value

This requires some small refactors to silence false-positive warnings.

Also, expand the bugprone-unused-return-value.CheckedReturnTypes option
to include util::Result, and util::Expected.

src/wallet/test/fuzz/spend.cpp

@@ -98,7 +98,7 @@ FUZZ_TARGET(wallet_create_transaction, .init = initialize_setup)
 
     std::optional<unsigned int> change_pos;
     if (fuzzed_data_provider.ConsumeBool()) change_pos = fuzzed_data_provider.ConsumeIntegral<unsigned int>();
-    (void)CreateTransaction(*fuzzed_wallet.wallet, recipients, change_pos, coin_control);
+    [[maybe_unused]] auto _{CreateTransaction(*fuzzed_wallet.wallet, recipients, change_pos, coin_control)};
 }
 } // namespace
 } // namespace wallet