[SYCL] Don't use zstd context across threads.

[uditagarwal97]

ZSTDCompressor holds zstd context as its only data members. The idea behind GetSingletonInstance() method was to re-use these contexts for subsequent compression and decompressions. Re-using context across (de)compression reduces system memory usage.

However, zstd contexts are not meant to be used concurrently, therefore, this PR makes ZSTDCompressor object thread-local, instead of static.
Relevant excerpt from zstd doc (https://facebook.github.io/zstd/zstd_manual.html):

When decompressing many times,
it is recommended to allocate a context only once,
and re-use it for each successive compression operation.
This will make workload friendlier for system's memory.
Use one context per thread for parallel execution.

[Copilot]

Pull Request Overview

This PR addresses thread safety issues with ZSTD compression by making the ZSTDCompressor singleton thread-local instead of global static. The change prevents concurrent access to ZSTD contexts, which are not thread-safe according to the ZSTD documentation.

• Changed ZSTDCompressor::GetSingletonInstance() to use thread_local storage instead of static
• Added comprehensive concurrent compression/decompression test with 20 threads
• Ensures each thread has its own ZSTD context while maintaining the singleton pattern benefits

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
sycl/source/detail/compression.hpp	Modified singleton to use thread_local storage for thread safety
sycl/unittests/compression/CompressionTests.cpp	Added concurrent test to verify thread-safe compression/decompression