[wallet] Validate inputs added with add_foreign_utxo

[ValuedMammal]

The wallet should check that a foreign utxo is indeed foreign before proceeding to build a transaction with it.

See a related discord comment

https://discord.com/channels/753336465005608961/978744259693916230/1326206434144288821

[nymius]

I think this check is necessary. We could filter out foreign utxos when their associated outpoints are found in the tx graph of the wallet.