qt: Avoid crash on startup if int specified in settings.json

Fix GUI startup crash reported by Rspigler in
https://github.com/bitcoin/bitcoin/issues/24457 that happens if
settings.json contains an integer value for any of the configuration
options which GUI settings can currently clash with (-dbcache, -par,
-spendzeroconfchange, -signer, -upnp, -natpmp, -listen, -server, -proxy,
-proxy, -onion, -onion, -lang, and -prune).

Fix is a one-line change in ArgsManager::GetArg.

src/qt/test/optiontests.cpp

@@ -15,15 +15,15 @@
 //! Entry point for BitcoinApplication tests.
 void OptionTests::optionTests()
 {
-    // Test regression https://github.com/bitcoin/bitcoin/issues/24457. Check
-    // if setting an integer prune value causes an exception to be thrown in
-    // the OptionsModel constructor.
+    // Test regression https://github.com/bitcoin/bitcoin/issues/24457. Ensure
+    // that setting integer prune value doesn't cause an exception to be thrown
+    // in the OptionsModel constructor
     gArgs.LockSettings([&](util::Settings& settings) {
         settings.forced_settings.erase("prune");
         settings.rw_settings["prune"] = 3814;
     });
     gArgs.WriteSettingsFile();
-    QVERIFY_EXCEPTION_THROWN(OptionsModel{}, std::runtime_error);
+    OptionsModel{};
     gArgs.LockSettings([&](util::Settings& settings) {
         settings.rw_settings.erase("prune");
     });

src/test/getarg_tests.cpp

@@ -98,21 +98,21 @@ BOOST_AUTO_TEST_CASE(setting_args)
 
     set_foo(99);
     BOOST_CHECK_EQUAL(args.GetSetting("foo").write(), "99");
-    BOOST_CHECK_THROW(args.GetArg("foo", "default"), std::runtime_error);
+    BOOST_CHECK_EQUAL(args.GetArg("foo", "default"), "99");
     BOOST_CHECK_EQUAL(args.GetIntArg("foo", 100), 99);
     BOOST_CHECK_THROW(args.GetBoolArg("foo", true), std::runtime_error);
     BOOST_CHECK_THROW(args.GetBoolArg("foo", false), std::runtime_error);
 
     set_foo(3.25);
     BOOST_CHECK_EQUAL(args.GetSetting("foo").write(), "3.25");
-    BOOST_CHECK_THROW(args.GetArg("foo", "default"), std::runtime_error);
+    BOOST_CHECK_EQUAL(args.GetArg("foo", "default"), "3.25");
     BOOST_CHECK_THROW(args.GetIntArg("foo", 100), std::runtime_error);
     BOOST_CHECK_THROW(args.GetBoolArg("foo", true), std::runtime_error);
     BOOST_CHECK_THROW(args.GetBoolArg("foo", false), std::runtime_error);
 
     set_foo(0);
     BOOST_CHECK_EQUAL(args.GetSetting("foo").write(), "0");
-    BOOST_CHECK_THROW(args.GetArg("foo", "default"), std::runtime_error);
+    BOOST_CHECK_EQUAL(args.GetArg("foo", "default"), "0");
     BOOST_CHECK_EQUAL(args.GetIntArg("foo", 100), 0);
     BOOST_CHECK_THROW(args.GetBoolArg("foo", true), std::runtime_error);
     BOOST_CHECK_THROW(args.GetBoolArg("foo", false), std::runtime_error);

src/util/system.cpp

@@ -591,7 +591,7 @@ bool ArgsManager::IsArgNegated(const std::string& strArg) const
 std::string ArgsManager::GetArg(const std::string& strArg, const std::string& strDefault) const
 {
     const util::SettingsValue value = GetSetting(strArg);
-    return value.isNull() ? strDefault : value.isFalse() ? "0" : value.isTrue() ? "1" : value.get_str();
+    return value.isNull() ? strDefault : value.isFalse() ? "0" : value.isTrue() ? "1" : value.isNum() ? value.getValStr() : value.get_str();
 }
 
 int64_t ArgsManager::GetIntArg(const std::string& strArg, int64_t nDefault) const