fix: make the JSONCPP_USE_SECURE_MEMORY build compile and pass (#1709)

Building with JSONCPP_USE_SECURE_MEMORY=1 fails in three separate ways,
each hidden behind the previous one. No CI job builds this configuration,
which is why they have accumulated.

1. allocator.h calls RtlSecureZeroMemory on the _WIN32 branch without
   including <windows.h>, so MSVC fails with error C3861. Removing that
   branch lets the portable volatile std::fill_n path handle Windows.
   The same fill was zeroing n bytes rather than n * sizeof(T), so for
   any T wider than a char, most of the allocation was never wiped.

2. jsontestrunner declares a std::string where the surrounding code uses
   Json::String. The two are the same type only in default builds, so the
   test runner does not compile under secure memory.

3. CZString's move-assignment released a key with
   releasePrefixedStringValue, but CZString keys come from
   duplicateStringValue and carry no length prefix. The destructor
   already uses the matching releaseStringValue. Under the default
   allocator this mismatch is survivable; under SecureAllocator the
   suite segfaults.

Verified on MSVC 19.44, x64, C++17. Before: error C3861. With only the
first two fixed: builds, then jsoncpp_test SEGFAULT. With all three:
secure build compiles and 3/3 ctest suites pass. The default build is
unaffected and also 3/3.

Fixes #1399

Co-authored-by: Jordan Bayles <bayles.jordan@gmail.com>