Fix long double inf test under Valgrind. Fixes #5175

[RUSLoker]

What

ci_test_valgrind started failing after #3929 added long-double serialization coverage. The two failing assertions are:

CHECK(long_double_json( std::numeric_limits<long double>::infinity()).dump() == "null");
CHECK(long_double_json(-std::numeric_limits<long double>::infinity()).dump() == "null");

doctest reports:

values: CHECK( 1.18973149535723176509e+4932 == null )

This PR keeps the assertions but guards them with a runtime probe, so the section continues to validate the production behavior on every real platform while not failing under Valgrind:

volatile long double inf_probe = std::numeric_limits<long double>::infinity();
if (!std::isfinite(inf_probe))
{
    CHECK(long_double_json( std::numeric_limits<long double>::infinity()).dump() == "null");
    CHECK(long_double_json(-std::numeric_limits<long double>::infinity()).dump() == "null");
}

Only tests/src/unit-serialization.cpp is touched (+15/−2). No library code is modified.

Why

The library itself is correct — dump_float() already calls if (!std::isfinite(x)) { write "null"; return; }. The failure is in Valgrind, not in the JSON code.

Valgrind has had incomplete 80-bit x87 long-double support since 2009 (Valgrind bug #197915, status ASSIGNED — Valgrind tracks its bugs on bugs.kde.org). On every long-double operation Valgrind round-trips through 64-bit double, and +inf/-inf come back out looking like LDBL_MAX. Reproduced locally with g++ 13.3 / valgrind 3.22.0 on Ubuntu 24.04 — the bit pattern of std::numeric_limits<long double>::infinity() is preserved in memory, but std::isfinite() returns true, std::isinf() returns false, and snprintf("%Lg", …) prints the LDBL_MAX decimal instead of "inf". That's exactly the symptom in the issue.

Why a runtime probe (and why volatile)

• The guard asks the only question that actually matters here — "does this platform's std::isfinite() recognize long-double infinity?" — so it adapts to any toolchain that exhibits the same defect, with no dependency on Valgrind-specific headers or build flags. The test file stays standard C++.
• volatile is load-bearing. Without it, std::isfinite of a constexpr infinity is folded to false at compile time and the guard becomes dead code. volatile forces a runtime read through the same FP path dump_float() exercises, so the probe truly reflects platform behavior.
• The NaN assertion still runs everywhere — Valgrind handles long-double NaN correctly; only +/-inf is mishandled.
• A TODO next to the guard links to the upstream Valgrind bug so the workaround can be removed once Valgrind ships proper 80-bit support and the project bumps its minimum Valgrind version.

Verification

Tested against the matrix of compilers/standards the CI exercises (Ubuntu 24.04 / GCC 13.3 / Clang 18.1 / Valgrind 3.22.0):

Configuration	Normal	Under Valgrind
GCC, C++11/14/17/20/23	31/31 ✓	29/29 ✓ (2 inf assertions probe-skipped, NaN still run)
GCC, project's strict warning set including -Wvolatile, -Werror	clean	—
Clang 18 / C++20	31/31 ✓	unchanged from before this PR (pre-existing round-trip issue, unrelated; ci_test_valgrind uses GCC)
ctest -L valgrind -R test-serialization_cpp11_valgrind (CI target)	—	Passed

Full test-serialization_cpp11 under Valgrind: 7/7 cases pass, 105/105 assertions pass, no Memcheck errors.

---

• The changes are described in detail, both the what and why.
• If applicable, an existing issue is referenced. — Test "dump for basic_json with long double number_float_t" fails when run with Valgrind #5175
• The Code coverage remained at 100%. A test case for every new line of code. — N/A; test-only change, no production lines added.
• If applicable, the documentation is updated. — N/A.
• The source code is amalgamated by running make amalgamate. — N/A; no headers touched.

Read the Contribution Guidelines for detailed information.

[nlohmann]

Looks good to me.

[nlohmann]

There are 2 Clang-Tidy warnings to be fixed:

/__w/json/json/tests/src/unit-serialization.cpp:353:9: error: missing username/bug in TODO [google-readability-todo,-warnings-as-errors]
  353 |         // TODO: remove this guard once Valgrind's 80-bit long double support
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |         // TODO(unknown): remove this guard once Valgrind's 80-bit long double support
/__w/json/json/tests/src/unit-serialization.cpp:357:9: error: variable 'inf_probe' of type 'volatile long double' can be declared 'const' [misc-const-correctness,-warnings-as-errors]
  357 |         volatile long double inf_probe = std::numeric_limits<long double>::infinity();
      |         ^
      |                              const 
431 warnings generated.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks for the quick fix!