Fix allocation failure during JSON destruction

[michaelsam94]

This PR fixes #5135 by preventing allocation failures from the temporary destruction stack from escaping basic_json's noexcept destructor.

The existing iterative destruction path is kept unchanged for the normal case. It is now wrapped with the library's JSON_TRY / JSON_CATCH macros; if growing the heap-allocated stack throws, destruction falls back to the regular array/object cleanup below instead of allowing the exception to cross the destructor boundary and call std::terminate.

A regression test in unit-regression2.cpp injects a one-shot global allocation failure while destroying a structured JSON value, verifying that the destructor consumes the failing stack allocation path and returns normally.

• 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.

Local verification:

/usr/bin/c++ -std=c++11 -Iinclude -Itests/src -Itests/thirdparty/doctest tests/src/unit.cpp tests/src/unit-regression2.cpp -o build-local/unit-regression2 && ./build-local/unit-regression2 --test-case='*5135*'
./build-local/unit-regression2
/usr/bin/c++ -std=c++11 -Iinclude -Itests/src -Itests/thirdparty/doctest tests/src/unit.cpp tests/src/unit-allocator.cpp -o build-local/unit-allocator && ./build-local/unit-allocator
printf '#include <nlohmann/json.hpp>\nint main(){ nlohmann::json j = nlohmann::json::array({1,2,3}); return j.size() == 3 ? 0 : 1; }\n' | /usr/bin/c++ -std=c++11 -Isingle_include -x c++ - -o build-local/single-smoke && ./build-local/single-smoke
printf '#include <nlohmann/json.hpp>\nint main(){ nlohmann::json j = nlohmann::json::array({1,2,3}); return j.size() == 3 ? 0 : 1; }\n' | /usr/bin/c++ -std=c++11 -fno-exceptions -DJSON_NOEXCEPTION -Iinclude -x c++ - -o build-local/noexceptions-smoke && ./build-local/noexceptions-smoke
git diff --check

I could not run the full CMake suite locally because cmake is not available on this machine's PATH.

[nlohmann]

I just merged a test which caused a conflict. Sorry, please rebase.

[michaelsam94]

Rebased onto current develop (acf076a) and resolved the test conflict by keeping the newly merged #4320 regression and placing the #5135 regression after it.\n\nRe-ran local checks:\n\n- unit-regression2: 9 test cases, 128 assertions passed\n- unit-allocator: 3 test cases, 17 assertions passed\n- single-header smoke compile\n- no-exceptions smoke compile\n- git diff --check