Fix Clang deprecation warning for json_pointer operator== with ordered_json

[nlohmann]

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Fixes #5288.

ordered_json uses a transparent comparator (std::equal_to<>). is_comparable (detail/meta/type_traits.hpp) previously guarded the Compare(A, B) well-formedness checks with a flat && chain, with !is_json_pointer_of<A, B>::value as the first operand (added for #4621). That guard doesn't stop the compiler from forming the later operands: std::is_constructible<decltype(std::declval<Compare>()(std::declval<A>(), std::declval<B>()))> is a separately-named class template, so its decltype is substituted regardless of whether the first operand is false — plain && chains of independently-named templates aren't a true lazy short-circuit (unlike std::conjunction).

That instantiates std::equal_to<>::operator()(string, json_pointer), whose noexcept(noexcept(t == u)) clause (in libstdc++'s stl_function.h) resolves t == u to the deprecated json_pointer/string operator== (there's no non-deprecated overload for that pair). Clang warns on that unevaluated-context use with -Wdeprecated-declarations; GCC does not warn in this particular case, which is why the issue only reproduces on Clang.

The fix splits is_comparable so the Compare(A, B) checks live in a separate helper (is_comparable_no_json_pointer) that is only ever referenced from the specialization selected when is_json_pointer_of<A, B> is false (dispatched via a plain bool template parameter). This means the problematic decltype is only ever written — and thus only ever instantiated — when A/B are not a json_pointer/string pair, regardless of compiler.

• The changes are described in detail, both the what and why.
• If applicable, an existing issue is referenced.
• The Code coverage remained at 100%. A test case for every new line of code.
• If applicable, the documentation is updated.
• The source code is amalgamated by running make amalgamate.

Testing

• Reproduced the warning from Deprecation warning for operator== when using ordered_json when building with clang #5288 with both Clang 18 and GCC 13 on develop before the fix (Clang warns, GCC doesn't).
• After the fix, both compilers are warning-free (-Wdeprecated-declarations) across -std=c++11/14/17/20, for both json and ordered_json, using .at()/.contains() with both json_pointer and std::string keys.
• Ran the unit-json_pointer, unit-comparison, unit-ordered_json, and unit-ordered_map unit test suites (4478 assertions) against the patched header — all pass.
• No new test case was added since this is a compiler-warning-only fix with no behavioral change; happy to add one if there's a preferred way to assert "no deprecation warning" in this test suite.

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