Improve performance of WebSocketMessageHandler

[Andrew Arnott (AArnott)]

Fixes#1234

[Copilot]

Pull Request Overview

This PR improves the performance of the WebSocketMessageHandler by optimizing memory allocation in the WriteCoreAsync method. It modifies the Sequence<byte> constructor to use a shared array pool and sets a minimum span length hint.

• Uses ArrayPool<byte>.Shared instead of default memory allocation for the sequence builder
• Sets MinimumSpanLength property to this.sizeHint to optimize buffer sizing