Add conversation pause and resume controls

[kraenhansen]

Summary

• Add pause()/resume() to conversations, with voice sessions interrupting playback, muting input, silencing output, and sending periodic user activity while paused.
• Restore previous mic mute and volume settings on resume via pause-scoped resume handlers, and drop incoming audio while paused through explicit conversation audio handling plus output-level playback enablement.
• Use LiveKit remote audio track controls for WebRTC playback gating instead of only pausing attached DOM elements.
• Expose pause state through conversation.isPaused(), React useConversationPause(), and the combined useConversation() return.
• Expose pause/resume through React conversation controls and add a changeset.

Testing

• pnpm --filter @elevenlabs/client test --run src/index.test.ts
• pnpm --filter @elevenlabs/client test --run src/utils/WebRTCConnection.test.ts
• pnpm --filter @elevenlabs/client test --run
• pnpm --filter @elevenlabs/client build:esm
• pnpm --filter @elevenlabs/react test --run
• pnpm --filter @elevenlabs/react build:esm
• Pre-commit hook ran full repo lint/build/check-types successfully on final commit.