Micro-optimize TGUI_CREATE_MESSAGE (#59142)

code/modules/unit_tests/_unit_tests.dm

@@ -88,6 +88,7 @@
 #include "subsystem_init.dm"
 #include "surgeries.dm"
 #include "teleporters.dm"
+#include "tgui_create_message.dm"
 #include "timer_sanity.dm"
 #include "unit_test.dm"
 #include "wizard.dm"

code/modules/unit_tests/tgui_create_message.dm

@@ -0,0 +1,28 @@
+/// Test that `TGUI_CREATE_MESSAGE` is correctly implemented
+/datum/unit_test/tgui_create_message
+
+/datum/unit_test/tgui_create_message/Run()
+	var/type = "something/here"
+	var/list/payload = list(
+		"name" = "Terry McTider",
+		"heads_caved" = 100,
+		"accomplishments" = list(
+			"nothing",
+			"literally nothing",
+			list(
+				"something" = "just kidding",
+			),
+		),
+	)
+
+	var/message = TGUI_CREATE_MESSAGE(type, payload)
+
+	// Ensure consistent output to compare by performing a round-trip.
+	var/output = json_encode(json_decode(url_decode(message)))
+
+	var/expected = json_encode(list(
+		"type" = type,
+		"payload" = payload,
+	))
+
+	TEST_ASSERT_EQUAL(expected, output, "TGUI_CREATE_MESSAGE didn't round trip properly")