wallet: read wallet txs and note data by reference in more RPCs

z_listreceivedbyaddress copied a whole CWalletTx per returned note to call
GetTxTime(). z_viewtransaction copied the previous tx of each own spend,
CWalletTx::DecryptSaplingNote copied the output and the note data (witness
list included) to read one field, and zsTxSpendsToJSON copied the note
data. All of them now bind const references to the same elements. The
lookups that throw or assert still do, and output is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/wallet/rpcwallet.cpp

@@ -4696,7 +4696,7 @@ UniValue z_listreceivedbyaddress(const UniValue& params, bool fHelp, const CPubK
             }
             obj.push_back(Pair("outindex", (int)entry.op.n));
             obj.push_back(Pair("rawconfirmations", entry.confirmations));
-            auto wtx = pwalletMain->mapWallet.at(entry.op.hash); //.ToString());
+            const CWalletTx& wtx = pwalletMain->mapWallet.at(entry.op.hash); //.ToString());
             obj.push_back(Pair("time", wtx.GetTxTime()));
 
             obj.push_back(Pair("confirmations", dpowconfs));
@@ -4949,7 +4949,7 @@ UniValue z_viewtransaction(const UniValue& params, bool fHelp, const CPubKey& my
             continue;
         }
         auto op        = res->second;
-        auto wtxPrev   = pwalletMain->mapWallet.at(op.hash);
+        const CWalletTx& wtxPrev = pwalletMain->mapWallet.at(op.hash);
         auto decrypted = wtxPrev.DecryptSaplingNote(op).get();
         auto notePt    = decrypted.first;
         auto pa        = decrypted.second;