#3165 Make MemoryAllocator limits configurable and extensible

Prerequisites

• I have written a descriptive pull-request title
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• I have provided test coverage for my change (where applicable)

Description

The allocation limits on MemoryAllocator were internal with no public way to raise the 1 GB single-buffer default, and ApplyOptions was not callable from external subclasses, which blocked custom allocators.

• Expose MemoryGroupAllocationLimitBytes, SingleBufferAllocationLimitBytes, and AccumulativeAllocationLimitBytes with validated protected setters.
• Make ApplyOptions protected so derived allocators can accept options.
• Add MemoryAllocatorOptions.SingleBufferAllocationLimitMegabytes to raise the contiguous buffer cap up to 2047 MB without a custom allocator.
• Add public-api tests that implement a complete external allocator.
• Add behavior tests for the new option and its validation.