feat(client): replace ElevenLabs entry point with ConversationClient

- ConversationClient (new, ObservableObject) replaces the static
  ElevenLabs.startConversation(...) factories and process-wide
  ElevenLabs.configure(_:)/ElevenLabsConfiguration -- it's a reusable,
  long-lived object (init(callbacks:), start(auth:config:) async throws,
  endConversation(), reset()) instead of a one-shot factory returning a
  Conversation. ElevenLabs.version (public) has no replacement --
  SDKVersion.version is internal-only now.
- Conversation is no longer public -- it's an internal session type
  ConversationClient owns per start() call. Its new AsyncStream-serialized
  event pipeline (eventStreamContinuation/eventConsumerTask) replaces
  direct closure dispatch from the connection manager. toggleMute/
  setMuted/isMuted are renamed toggleMicMute/setMicMuted/isMicMuted, and
  gain a symmetric toggleAgentMute/setAgentMuted/isAgentMuted; calling
  the mic-mute methods while disconnected is now a lenient no-op instead
  of throwing .notConnected. inputTrack/agentAudioTrack (raw LiveKit
  track access) and audioDevices/selectedAudioDeviceID (device
  enumeration) are removed with no replacement. sendToolResult's generic
  Encodable overload is gone -- only the Any-based overload remains, so
  a plain Encodable struct now throws ConversationError.invalidToolResult
  instead of JSON-encoding automatically.
- CallInfo (agentId-carrying struct) is deleted -- ConversationState no
  longer exposes the connected agent's id at all.
- Removes the dead onRawMessage plumbing that shipped with this v4 import:
  both connection managers declared it and Conversation.messageSource(for:)
  existed to feed it, but nothing ever wired connectionManager.onRawMessage
  to actually fire, so the documented public ConversationCallbacks.onMessage
  callback could never fire either. Removed both rather than ship a
  callback that silently never calls back; confirmed via a test
  (testRawMessageCallbackReceivesSourceAndRawJSON) that failed exactly
  this way before the fix.
- Test updates are almost entirely mechanical renames to match the above
  (ConversationOptions -> ConversationCallbacks/ConversationConfig split,
  .active(CallInfo) -> .connected, ElevenLabs.* -> ConversationClient/
  ConversationAuth/ElevenLabsEndpoints), plus one real fix: the hardcoded
  SDK-version string in ElevenLabsSDKTests was still "3.2.0", stale
  against the intentional 3.2.2 bump already on this branch.
  ConversationConfigTests/EventSerializerTests/ConversationEventHandlerTests
  gained real new coverage for the config/event changes earlier in this
  stack (see their respective commits). Coverage lost with no direct
  replacement: testSendToolResultEncodesEncodableResult (tracks the
  Encodable-overload removal above) and
  testTextOnlyStartDisconnectsPreviousActiveManagerBeforeSwitchingTransports
  (WebRTC->WebSocket teardown-before-switch ordering).

Full suite: 154 tests, 0 failures.

Author: renal128