feat: add MaintainFramerateAndResolution to DegradationPreference enum

[xianshijing-lk]

Summary

Aligns the DegradationPreference enum with WebRTC M144 changes:

• Add MAINTAIN_FRAMERATE_AND_RESOLUTION as the new recommended value
• Deprecate DISABLED (maps to MAINTAIN_FRAMERATE_AND_RESOLUTION for backwards compatibility)

Reference: https://github.com/webrtc-sdk/webrtc/blob/m144_release/api/rtp_parameters.h#L125

Changes

• room.proto: Add DEGRADATION_PREFERENCE_MAINTAIN_FRAMERATE_AND_RESOLUTION enum value
• webrtc-sys: Rename Disabled to MaintainFramerateAndResolution in the repr(i32) enum
• libwebrtc: Add MaintainFramerateAndResolution variant, deprecate Disabled as a const alias
• livekit-ffi: Update conversion to handle both old DISABLED and new MAINTAIN_FRAMERATE_AND_RESOLUTION proto values

Backwards Compatibility

• Existing code using DISABLED will continue to work (mapped to MaintainFramerateAndResolution)
• FFI clients can use either value, both map to the same internal behavior

Test plan

• cargo build -p livekit-ffi passes
• cargo build -p livekit passes
• cargo test -p livekit --lib -- degradation passes

🤖 Generated with Claude Code

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Changeset

The following package versions will be affected by this PR:

Package	Bump
libwebrtc	minor
livekit	minor
livekit-ffi	minor
webrtc-sys	minor