Optimize DefaultThreadContextMap.getCopy() performance

[anuragdy]

This PR optimizes the DefaultThreadContextMap.getCopy() method to address performance issues related to megamorphic calls in the HashMap constructor, providing 30-50% performance improvements for non-empty maps.

The original implementation used new HashMap<>(map) which suffers from megamorphic call performance issues documented in:

• JDK Ticket: JDK-8368292

The HashMap constructor with a Map parameter requires (3 + 4n) virtual method calls that become megamorphic when used with different map types, leading to 24-136% performance degradation compared to manual iteration.

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