* rtc: apply simulcast rids from SDP for WHIP (one-shot) publishers
In one-shot signalling mode HandleRemoteDescription set the remote offer
directly and never called OnSetRemoteDescriptionOffer, so
updateRidsFromSDP did not run for WHIP participants. Every simulcast layer
then mapped to spatial layer 0 and all but the first were rejected with
'duplicate layer'.
* rtc: test that a one-shot simulcast offer applies rids to the pending track
* rtc: use h/q rids in one-shot simulcast test, cover single-layer offer
Name the simulcast test layers medium/h and low/q per convention, and add
TestOneShotSingleLayerOfferUnchanged to check a one-shot offer without rids
still yields a single layer with rid "" at spatial layer 0.