test: cover JSON_NO_IO, JSON_THROW/TRY/CATCH_USER, JSON_SKIP_LIBRARY_VERSION_CHECK, and JSON_DisableEnumSerialization in CI (#5423)

These four supported configuration macros were never actually compiled
anywhere in the test matrix:

- JSON_NO_IO and the JSON_THROW_USER/JSON_TRY_USER/JSON_CATCH_USER trio
  are exercised together in a new tests/src/unit-no_io_and_user_exceptions.cpp,
  which is automatically picked up by the existing unit-*.cpp test glob and
  thus built across the whole standard test matrix.
- JSON_SKIP_LIBRARY_VERSION_CHECK is exercised by a new, dedicated
  tests/src/skip_library_version_check.cpp, compiled directly by the new
  ci_test_skiplibraryversioncheck target in cmake/ci.cmake: the scenario it
  simulates (mixing two differently-versioned inclusions of the library)
  unavoidably triggers the compiler's own "macro redefined" warning, which
  would fail under the library's own -Weverything/-Werror unit test matrix
  for a reason unrelated to the macro under test.
- JSON_DisableEnumSerialization already had #if-guarded tests in several
  unit-*.cpp files (from #4384), but no CMake target ever actually set the
  JSON_DisableEnumSerialization CMake option, so that guarded code was never
  compiled. Add ci_test_disableenumserialization, mirroring the existing
  ci_test_noimplicitconversions/ci_test_noglobaludls targets. Building the
  full test suite with this option on surfaced one real, narrow gap: get<T>()
  on std::vector<std::byte> (used by unit-regression2.cpp's custom BinaryType
  tests) relies on std::byte being handled via enum serialization, so add the
  same #if-guard convention to the two affected SECTIONs there.

Both new CI targets are added to the ci_cmake_options matrix in
.github/workflows/ubuntu.yml, alongside the existing ci_test_* targets.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/skip_library_version_check.cpp

@@ -0,0 +1,61 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// Standalone compile-and-run check for the JSON_SKIP_LIBRARY_VERSION_CHECK
+// configuration macro, which (per #5423) was never exercised anywhere in the
+// test matrix.
+//
+// include/nlohmann/detail/abi_macros.hpp normally emits a #warning if
+// NLOHMANN_JSON_VERSION_MAJOR/MINOR/PATCH are already defined (as they would
+// be by an earlier inclusion of a different version of the library) with
+// values that mismatch the version about to be defined -- unless
+// JSON_SKIP_LIBRARY_VERSION_CHECK is defined, in which case the check (and
+// that #warning) is skipped.
+//
+// This file deliberately is not named tests/src/unit-*.cpp: it is compiled
+// directly (with a modest, non-strict warning set) by the dedicated
+// ci_test_skiplibraryversioncheck target in cmake/ci.cmake, rather than being
+// folded into the library's own -Weverything/-Werror unit test matrix. That
+// is because the scenario simulated here -- mixing two different, already
+// differently-versioned inclusions of the library in one translation unit --
+// unavoidably also triggers the *compiler's own* "macro redefined" warning,
+// independent of (and unaffected by) JSON_SKIP_LIBRARY_VERSION_CHECK, which
+// only ever silences the library's own #warning. Building this file under
+// -Weverything -Werror would therefore fail for a reason unrelated to the
+// macro under test.
+#define NLOHMANN_JSON_VERSION_MAJOR 0
+#define NLOHMANN_JSON_VERSION_MINOR 0
+#define NLOHMANN_JSON_VERSION_PATCH 0
+
+#define JSON_SKIP_LIBRARY_VERSION_CHECK 1
+
+#include <nlohmann/json.hpp>
+
+int main()
+{
+    // reaching this point at all already proves that the mismatched,
+    // pre-defined version macros above did not stop compilation -- which is
+    // exactly what JSON_SKIP_LIBRARY_VERSION_CHECK is for. The library must
+    // also still be fully usable.
+    const nlohmann::json j = {{"a", 1}, {"b", {1, 2, 3}}};
+    if (j.dump() != "{\"a\":1,\"b\":[1,2,3]}")
+    {
+        return 1;
+    }
+
+    // include/nlohmann/detail/abi_macros.hpp unconditionally (re)defines the
+    // version macros to the library's real, current version right after the
+    // (here, skipped) mismatch check, regardless of the deliberately wrong
+    // stand-in values defined above.
+    if (NLOHMANN_JSON_VERSION_MAJOR == 0 && NLOHMANN_JSON_VERSION_MINOR == 0 && NLOHMANN_JSON_VERSION_PATCH == 0)
+    {
+        return 1;
+    }
+
+    return 0;
+}