[Interop 2026] Apply simulcast scaling defaults

Apply scale_resolution_down_by defaults to video encodings passed to
AddTransceiver. If any encoding provides a value, fill missing values
with 1.0 before trimming the list. Otherwise, assign descending powers
of two after trimming.

This aligns sender parameters with the WebRTC addTransceiver algorithm
and fixes two Interop 2026 WPT subtests in Chromium.

R=perkj@webrtc.org, tommi@webrtc.org

Bug: chromium:344943229
Change-Id: I490175e44f1b840e537d4e24718ca7cbef45389a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/497360
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48439}