[FCE-2938] Re-enable simulcast variant switching

[sgfn]

Description

• Re-enables simulcast in webrtc-client
• Adds a new useSetTargetTrackEncoding hook in react-client
• Propagates the existing per-track encodingChanged event up to the client-level MessageEvents in ts-client
• Updates the minimal-react example to enable simulcast and render variant selection buttons

Motivation and Context

FCE-2938
Simulcast was deliberately disabled pending manual track selection support.

Documentation impact

• Documentation update required
• Documentation updated in another PR
• No documentation update required

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)

[linear]

FCE-2938 Re-enable simulcast

[Karolk99]

As discussed in the office, we don't need this event at the moment. Working example and setVideoQuality function instead of setTrackTargetVariant is enough for now. Also, you can add a test for switching layers and simulcast as another integration test.

[czerwiukk]

tests failing due to staging outage