wallet: disable sending to silent payment address

Have `IsValidDestination` return false for silent payment destinations
and set an error string when decoding a silent payment address.

This prevents anyone from sending to a silent payment address before
sending is implemented in the wallet, but also allows the functions to
be used in the unit testing famework.

src/addresstype.cpp

@@ -160,7 +160,9 @@ public:
     bool operator()(const PubKeyDestination& dest) const { return false; }
     bool operator()(const PKHash& dest) const { return true; }
     bool operator()(const ScriptHash& dest) const { return true; }
-    bool operator()(const V0SilentPaymentDestination& dest) const { return true; }
+    // silent payment addresses are not valid until sending support has been implemented
+    // TODO: set this to true once sending is implemented
+    bool operator()(const V0SilentPaymentDestination& dest) const { return false; }
     bool operator()(const WitnessV0KeyHash& dest) const { return true; }
     bool operator()(const WitnessV0ScriptHash& dest) const { return true; }
     bool operator()(const WitnessV1Taproot& dest) const { return true; }

src/key_io.cpp

@@ -174,6 +174,11 @@ CTxDestination DecodeDestination(const std::string& str, const CChainParams& par
             }
             CPubKey scan_pubkey{data.begin(), data.begin() + CPubKey::COMPRESSED_SIZE};
             CPubKey spend_pubkey{data.begin() + CPubKey::COMPRESSED_SIZE, data.begin() + 2*CPubKey::COMPRESSED_SIZE};
+            // This is a bit of a hack to disable silent payments until sending is implemented. The reason we return a V0SilentPaymentDestination
+            // while also setting an error message is so that we can use DecodeDestination in the unit tests, but also have `validateaddress` fail
+            // when passed a silent payment address
+            // TODO: remove this error_str once sending support is implemented
+            error_str = strprintf("This is a valid Silent Payments v0 address, but sending support is not yet implemented.");
             return V0SilentPaymentDestination{scan_pubkey, spend_pubkey};
         }
         // Bech32 decoding