fix(react-client): await pending addTrack to fix track ops race

[Critteros]

Description

getCurrentTrackId now awaits the in-flight addTrack promise before
returning, so callers always see the resolved remote track id once the
track is registered.

• Add connectionPromiseRef storing the active addTrack job
• Make getCurrentTrackId async and await pending job
• Update callers (replaceTrack, refreshStreamedTrack,
  toggleMute, toggleDevice) to await the new async signature
• Await pauseStreaming in toggleDevice

Motivation and Context

startStreaming set currentTrackIdRef.current only after addTrack
resolved. If the user invoked toggleMute, replaceTrack, or
toggleDevice during that window, getCurrentTrackId returned null
and the operation silently no-op'd (mute toggles ignored, replace
skipped). Awaiting the pending promise removes the race.

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)

[czerwiukk]

rebase and check if the tests pass, code itself looks good