Disallow RTP header extension ID of 0

An extension ID of 0 is invalid, but was being silently bypassed during
validation, causing downstream fuzzer crashes. This regression was
introduced in https://webrtc-review.googlesource.com/c/src/+/453661.

This CL removes the id == 0 bypass in RtpTransport, ensuring that all
extension IDs are strictly verified to fall within the valid range (>=
RtpExtension::kMinId).

Unit tests have also been restored/added to verify that out-of-bounds
IDs at the lower limit are properly rejected.

Bug: chromium:506682780
Change-Id: I4f0a3f0a937740dcf865f7607c115baa33229f23
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/470920
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47676}