Implementing ScratchBufferAllocator

[TalZaccai]

ScratchBufferAllocator is similar to ScratchBufferBuilder (previously ScratchBufferManager), but instead of creating a new buffer and copying the previous data when the buffer has no sufficient space (and releasing the pointer to the previous buffer), it instead keeps a stack of the previous buffers and does not copy the previous data.
By doing so, ScratchBufferAllocator ensures that any previously created ArgSlices that have not been explicitly re-winded do not point to GCed memory.
The reason that both managers are currently necessary, is that ScratchBufferBuilder supports getting all the data in a continuous chunk of memory, where ScratchBufferAllocator does not.

[Copilot]

Pull Request Overview

This PR implements the new ScratchAllocationManager to manage memory allocations without copying existing data and updates all relevant usages from ScratchBufferManager to ScratchAllocationManager.

• Implement new ScratchAllocationManager with backup buffer reuse
• Update transaction, response, and custom command code to use ScratchAllocationManager
• Adjust unit tests to validate behavior of the new manager

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/Garnet.test/ScratchAllocationManagerTests.cs	Added tests to verify correct allocation, rewind, and reset behavior
libs/server/Transaction/TxnRespCommands.cs	Updated custom procedure invocation to use ScratchAllocationManager
libs/server/Transaction/TransactionManager.cs	Replaced scratch buffer manager with scratch allocation manager
libs/server/Resp/RespServerSession.cs	Initialized and reset the new ScratchAllocationManager alongside the old manager
libs/server/Custom/CustomTransactionProcedure.cs	Updated method calls to use ScratchAllocationManager
libs/server/Custom/CustomRespCommands.cs	Updated call to GetCustomTransactionProcedure to use the allocation manager
libs/server/Custom/CustomCommandManagerSession.cs	Revised method signature and assignment for scratch allocation manager
libs/server/ArgSlice/ScratchAllocationManager.cs	New implementation of ScratchAllocationManager with expanded buffer logic
libs/common/RefStack.cs	Updated to use new C# 12 array initialization for an empty stack