Document rationale for @unchecked Sendable in TestProxyTransport

[coderabbitai]

The TestProxyTransport class in Tests/AblyLiveObjectsTests/JS Integration Tests/TestProxyTransport.swift at line 57 is marked as @unchecked Sendable without explanation.

Please add a comment above the class declaration explaining why manual Sendable conformance is safe here, particularly regarding the thread-safety guarantees provided by the dispatch queue synchronization.

Related PR: #21
Comment: #21 (comment)
Requested by: lawrence-forooghian

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