Broadcast a signed Solana transaction

How you call this depends on whether the build returned an executionRef. Absent: we assembled the route, so send only signedTransaction and we broadcast it. Present: the route came from the meta-aggregator, an RFQ leg can involve a market maker with last-look rights, and it refuses a transaction altered after quoting, so it has to go back through it to land. Broadcasting that one yourself raises no error, it simply never confirms. Either way this is a relay, not custody: the transaction arrives already signed, so all this can do is deliver it or not.