Additionally treat Tx.nVersion as unsigned in joinpsbts

This gets its own release note callout, though doesn't appear to
violate the BIP as the BIP appears to be underspecified. We
probably want to update BIP 174 to mention how version numbers are
combined.

doc/release-notes-16525.md

@@ -4,4 +4,6 @@ RPC changes
 Exposed transaction version numbers are now treated as unsigned 32-bit integers
 instead of signed 32-bit integers. This matches their treatment in consensus
 logic. Versions greater than 2 continue to be non-standard (matching previous
-behavior of smaller than 1 or greater than 2 being non-standard).
+behavior of smaller than 1 or greater than 2 being non-standard). Note that
+this includes the joinpsbt command, which combines partially-signed
+transactions by selecting the highest version number.

src/rpc/rawtransaction.cpp

@@ -1566,7 +1566,7 @@ UniValue joinpsbts(const JSONRPCRequest& request)
         throw JSONRPCError(RPC_INVALID_PARAMETER, "At least two PSBTs are required to join PSBTs.");
     }
 
-    int32_t best_version = 1;
+    uint32_t best_version = 1;
     uint32_t best_locktime = 0xffffffff;
     for (unsigned int i = 0; i < txs.size(); ++i) {
         PartiallySignedTransaction psbtx;
@@ -1576,8 +1576,8 @@ UniValue joinpsbts(const JSONRPCRequest& request)
         }
         psbtxs.push_back(psbtx);
         // Choose the highest version number
-        if (psbtx.tx->nVersion > best_version) {
-            best_version = psbtx.tx->nVersion;
+        if (static_cast<uint32_t>(psbtx.tx->nVersion) > best_version) {
+            best_version = static_cast<uint32_t>(psbtx.tx->nVersion);
         }
         // Choose the lowest lock time
         if (psbtx.tx->nLockTime < best_locktime) {
@@ -1588,7 +1588,7 @@ UniValue joinpsbts(const JSONRPCRequest& request)
     // Create a blank psbt where everything will be added
     PartiallySignedTransaction merged_psbt;
     merged_psbt.tx = CMutableTransaction();
-    merged_psbt.tx->nVersion = best_version;
+    merged_psbt.tx->nVersion = static_cast<int32_t>(best_version);
     merged_psbt.tx->nLockTime = best_locktime;
 
     // Merge