rpc: convert settxfee to RPCHelpMan, fix below-minimum error path

Convert settxfee to the RPCHelpMan pattern landed in gridcoin-community#2923. As a
side effect, fix the below-minimum-amount UX bug: today the help
condition is `fHelp || params.size() < 1 || params.size() > 1 ||
AmountFromValue(params[0]) < nMinFee`, so a below-min amount throws
the help text wrapped as RPC_MISC_ERROR (-1) — useless to the caller.

Hoist the AmountFromValue and minimum-fee check out of the help
condition; below-min now throws JSONRPCError(RPC_INVALID_PARAMETER,
"txfee X below minimum Y") with both values formatted, so the caller
gets actionable feedback. The 0-arg path is unchanged in behavior:
IsValidNumArgs(0) returns false and the help is thrown, just as the
short-circuit `params.size() < 1` produced before.

Add a per-command help-render test in rpchelpman_tests.cpp asserting
that fHelp=true throws a runtime_error whose message contains the
expected RPCHelpMan markers.

Refs: gridcoin-community#2922

Author: PrestackI

src/rpc/blockchain.cpp

@@ -11,6 +11,7 @@
 #include "gridcoin/scraper/scraper_registry.h"
 #include "gridcoin/sidestake.h"
 #include "node/blockstorage.h"
+#include <rpc/util.h>
 #include <util/string.h>
 #include "gridcoin/mrc.h"
 #include "gridcoin/support/block_finder.h"
@@ -839,23 +840,34 @@ UniValue getdifficulty(const UniValue& params, bool fHelp)
 
 UniValue settxfee(const UniValue& params, bool fHelp)
 {
+    static const RPCHelpMan help{
+        "settxfee",
+        "Set the transaction fee per kB (rounded to the nearest 0.00000001 GRC). "
+        "Overwrites the paytxfee parameter.",
+        {
+            {"amount", RPCArg::Type::AMOUNT, RPCArg::Optional::NO,
+                "The transaction fee in GRC/kB."},
+        },
+        RPCResult{RPCResult::Type::BOOL, "", "Returns true if successful."},
+        RPCExamples{
+            HelpExampleCli("settxfee", "0.00001") +
+            HelpExampleRpc("settxfee", "0.00001")},
+    };
+    if (fHelp || !help.IsValidNumArgs(params.size()))
+        throw runtime_error(help.ToString());
+
     LOCK(cs_main);
 
     CTransaction txDummy;
+    const CAmount nMinFee = GetBaseFee(txDummy, GMF_SEND);
+    const CAmount amount = AmountFromValue(params[0]);
+    if (amount < nMinFee) {
+        throw JSONRPCError(RPC_INVALID_PARAMETER,
+            strprintf("txfee %s below minimum %s",
+                      FormatMoney(amount), FormatMoney(nMinFee)));
+    }
 
-    // Min Fee
-    CAmount nMinFee = GetBaseFee(txDummy, GMF_SEND);
-
-    if (fHelp || params.size() < 1 || params.size() > 1 || AmountFromValue(params[0]) < nMinFee)
-        throw runtime_error(
-                "settxfee <amount>\n"
-                "\n"
-                "<amount> is a real and is rounded to the nearest 0.00000001\n"
-                "\n"
-                "Sets the txfee for transactions\n");
-
-    nTransactionFee = AmountFromValue(params[0]);
-
+    nTransactionFee = amount;
     return true;
 }
 

src/test/rpchelpman_tests.cpp

@@ -12,6 +12,15 @@
 #include <stdexcept>
 #include <string>
 
+// Forward declarations of the Tier 2 commands under test. These must live in
+// the global namespace; if placed inside BOOST_AUTO_TEST_SUITE(...) they get
+// captured into the suite's namespace and fail to link against the
+// definitions in src/rpc/blockchain.cpp, src/rpc/net.cpp, src/wallet/rpcwallet.cpp.
+// Each command's converted body throws for fHelp=true before touching any
+// globals (vNodes, g_banman, pwalletMain, mapBlockIndex, locks), so calling
+// these with fHelp=true and an empty params array is safe in unit tests.
+UniValue settxfee(const UniValue& params, bool fHelp);
+
 BOOST_AUTO_TEST_SUITE(rpchelpman_tests)
 
 // Helper: build a minimal RPCHelpMan with one required string arg and one result.
@@ -319,4 +328,18 @@ BOOST_AUTO_TEST_CASE(tier1_throw_pattern)
     }
 }
 
+BOOST_AUTO_TEST_CASE(settxfee_help_renders)
+{
+    const UniValue params(UniValue::VARR);
+    try {
+        settxfee(params, /*fHelp=*/true);
+        BOOST_FAIL("expected runtime_error");
+    } catch (const std::runtime_error& e) {
+        const std::string what{e.what()};
+        BOOST_CHECK(what.find("settxfee") != std::string::npos);
+        BOOST_CHECK(what.find("amount") != std::string::npos);
+        BOOST_CHECK(what.find("Examples:") != std::string::npos);
+    }
+}
+
 BOOST_AUTO_TEST_SUITE_END()