chore: remove public LiveKit audio track re-exports

[renal128]

What

• Remove the public ElevenLabs.LocalAudioTrack / RemoteAudioTrack / AudioTrack LiveKit re-exports.
• Drop the unused import LiveKit from ElevenLabs.swift.
• Update Usage docs to point at ConversationAudioObserver instead of conversation.inputTrack / agentAudioTrack.
• Clarify that session track accessors are internal plumbing for observer attachment.

Why

Raw audio observation is now available through ConversationAudioObserver without exposing LiveKit types. These re-exports were the last public LiveKit type surface in the SDK.

Notes

• LiveKit remains an internal package dependency.
• Internal Conversation / connection-manager track accessors are unchanged.

Testing

• swiftformat 'Sources' 'Tests' --lint --strict
• Focused startup/client/audio tests
• swift test

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Note

Medium Risk
Breaking public API removal for apps that depended on LiveKit track types or published device lists; migration path is documented via ConversationAudioObserver.

Overview
Removes the last public LiveKit type surface from the SDK by deleting ElevenLabs re-exports of LocalAudioTrack, RemoteAudioTrack, and AudioTrack, and drops the unused import LiveKit from the public entry module.

Documentation now steers integrators toward Raw Audio Observers (ConversationAudioObserver on ConversationClient) instead of casting conversation.inputTrack / agentAudioTrack, with notes that didReceive(_:) runs off the main actor on borrowed PCM buffers.

Internal cleanup: Conversation no longer publishes audioDevices / selectedAudioDeviceID, and ConversationAudioManager no longer mirrors input device lists or onDeviceUpdate wiring—session track accessors remain internal for observer attachment only.

^{Reviewed by Cursor Bugbot for commit e2b81d5. Bugbot is set up for automated code reviews on this repo. Configure here.}