Correctly promote partial | promote(partial)

[saki7]

// previous implementation
BOOST_TEST(((PartialPartial{NaN, 1.0} <=> PartialPartial{NaN, 2.0}) == std::partial_ordering::unordered));
BOOST_TEST((comp(PartialPartial{NaN, 1.0}, PartialPartial{NaN, 2.0}) == std::partial_ordering::unordered));

// after this PR
BOOST_TEST(((PartialPartial{NaN, 1.0} <=> PartialPartial{NaN, 2.0}) == std::partial_ordering::unordered));
BOOST_TEST((comp(PartialPartial{NaN, 1.0}, PartialPartial{NaN, 2.0}) == std::partial_ordering::less));

@yaito3014 Can you confirm this is correct? If so please merge thanks!

[cppwarningnotifier]

Environment				C++23	C++26
ubuntu24.04	Clang	19	Debug	✅success	✅success
			Release	✅success	✅success
		20	Debug	✅success	✅success
			Release	✅success	✅success
	GCC	14	Debug	✅success	✅success
			Release	✅success	✅success
windows2022	MSVC	v142	Debug	✅success	✅success
			Release	✅success	✅success

[yaito3014]

LGTM