Add LocalTrackPublication.set(videoPublishOptions:) to update encodings without republishing

[tarsyang]

Motivation

Publish options are fixed at publish time.
LocalParticipant.set(source:enabled:) returns the existing
publication (unmute path) without applying newly passed
publishOptions, so the only way to change encoding parameters of a
published video track (e.g. lowering maxBitrate while the device is
under thermal pressure, then restoring it) is a full unpublish +
republish. That drops frames, churns the subscriber side, and re-runs
capture setup.

The SDK already updates sender encodings live when capture dimensions
change (recomputeSenderParameters()), so the machinery exists; it has
no public entry point.

API

public func set(videoPublishOptions options: VideoPublishOptions) throws

on LocalTrackPublication:

• Stores the new options on the track, re-computes encodings for the
  current capture dimensions, applies them to the RTP sender, and
  reports the updated layers to the server (sendUpdateVideoLayers),
  same as the dimensions-change path.
• The encoding layer structure is fixed at publish time: options that
  would produce a different layer count or rid set (e.g. toggling
  simulcast) throw .invalidState. Without the guard such options
  would deactivate every non-matching layer in
  recomputeSenderParameters()'s rid-matched merge.
• Throws .invalidState when the publication is not a local video
  track, the participant or room is no longer available, the RTP sender
  is not available, or capture dimensions are not resolved yet.
• Only per-layer encoding parameters (isActive,
  scaleResolutionDownBy, maxBitrate, maxFps) are re-applied;
  codec preferences, scalability mode, and degradationPreference
  remain as published (documented on the API).

Testing

• New e2e SetVideoPublishOptionsTests (local livekit-server --dev):
  maxBitrate of a published non-simulcast camera-source track changes
  from 1.7 Mbps to 1.2 Mbps on the live sender without republishing
  (same publication sid, still a single publication), and toggling
  simulcast is rejected with .invalidState.
• Verified the update test fails when the recompute step is removed.
• Built for iOS Simulator and macOS.

[devin-ai-integration]

✅ Devin Review: No Issues Found

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