wallet: also search taproot pubkeys in FillPSBT

When filling a PSBT, we search the listed pubkeys in order to determine
whether the current DescriptorScriptPubKeyMan could sign the transaction
even if it is not watching the scripts. With Taproot, the taproot
pubkeys need to be searched as well.

src/wallet/scriptpubkeyman.cpp

@@ -2180,6 +2180,19 @@ TransactionError DescriptorScriptPubKeyMan::FillPSBT(PartiallySignedTransaction&
                     *keys = Merge(*keys, *pk_keys);
                 }
             }
+            for (const auto& pk_pair : input.m_tap_bip32_paths) {
+                const XOnlyPubKey& pubkey = pk_pair.first;
+                for (unsigned char prefix : {0x02, 0x03}) {
+                    unsigned char b[33] = {prefix};
+                    std::copy(pubkey.begin(), pubkey.end(), b + 1);
+                    CPubKey fullpubkey;
+                    fullpubkey.Set(b, b + 33);
+                    std::unique_ptr<FlatSigningProvider> pk_keys = GetSigningProvider(fullpubkey);
+                    if (pk_keys) {
+                        *keys = Merge(*keys, *pk_keys);
+                    }
+                }
+            }
         }
 
         SignPSBTInput(HidingSigningProvider(keys.get(), !sign, !bip32derivs), psbtx, i, &txdata, sighash_type, nullptr, finalize);