test(libSceConvertKeycode): check the keyboard type lookup at both int32 limits at compile time

LookUp is constexpr, so static_assert evaluates it for INT32_MIN and INT32_MAX in a constant expression, where signed overflow is an error. With the previous 32-bit subtraction the test file no longer compiles ('overflow in constant expression'); the existing runtime case for 0x80000000 only showed the overflow under UBSan.

core/libs/tests/GuestConvertKeycode.cpp

@@ -15,6 +15,9 @@
 
 static void Require(bool value) { if (!value) std::abort(); }
 
+static_assert(ImeKeyboardType::LookUp(INT32_MIN, 0, 0) == 0);
+static_assert(ImeKeyboardType::LookUp(INT32_MAX, 0, 0) == 0);
+
 static void SetLanguage(const char* value) {
 #ifdef _WIN32
     Require(_putenv_s("ANYPS5_LANGUAGE", value != nullptr ? value : "") == 0);