feat(calling): add calling transport layer

[rsarika]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep per-callback state for connection listeners

LegacyMercuryTransportAdapter stores only one offlineHandler/onlineHandler pair on the adapter instance, so each onConnectionStateChange() call overwrites the previous callbacks while leaving the previous Mercury listeners registered. In any flow with multiple registrations (for example multiple CallingClient instances or repeated init/teardown), offConnectionStateChange() can only remove the most recently stored handlers, which leaves stale listeners active and can cause duplicate or misrouted connection-state handling.

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Reviewed commit: 45a6721a29

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[chatgpt-codex-connector]

Await adapter disposal before switching transport

setAdapter() invokes transportAdapter.dispose but does not await it, even though ICallingTransportAdapter.dispose is allowed to return Promise<void>. If a custom adapter performs async teardown (for example, closing a socket or unregistering listeners), the new adapter is installed immediately and both adapters can run concurrently, causing duplicate events or resource leaks during adapter swaps.

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