feat: add DegradationPreference enum (without deprecated Disabled)

Add a wrapper DegradationPreference enum that matches the Rust API:
- BALANCED: Balance between framerate and resolution degradation
- MAINTAIN_FRAMERATE: Degrade framerate to maintain resolution
- MAINTAIN_RESOLUTION: Degrade resolution to maintain framerate
- MAINTAIN_FRAMERATE_AND_RESOLUTION: Maintain both, drop frames before encoding

The deprecated 'Disabled' value is intentionally excluded since it has been
removed from WebRTC M144 and replaced with MaintainFramerateAndResolution.

Also regenerates proto files from livekit-ffi v0.12.68.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: xianshijing-lk

livekit-rtc/livekit/rtc/__init__.py

@@ -58,6 +58,7 @@
 )
 from .participant import (
     ConnectionQuality,
+    DegradationPreference,
     LocalParticipant,
     Participant,
     RemoteParticipant,
@@ -138,6 +139,7 @@
     "ConnectionQuality",
     "ConnectionState",
     "DataPacketKind",
+    "DegradationPreference",
     "TrackPublishOptions",
     "IceTransportType",
     "ContinualGatheringPolicy",