[Tests] Add VirtualMemory and PhysicalVirtualMemory edge case tests (#266)
VirtualMemory: 4 new tests covering unmapped address returns false,
zero-length operations, page-boundary access, and cross-gap failures.
PhysicalVirtualMemory: 5 new tests covering lazy commit on demand,
reserve-only GetPointer, unmapped GetPointer returns null, free-list
first-fit reuse, and coalescing both neighbours on middle-range free.
Total: 9 new tests, 18 passed (8 VirtualMemory + 5 PhysicalVirtualMemory
+ 5 GuestMemoryAllocator). No production code changes.
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