Invalidate RtpSender parameter cache on codec fallback/switch

The RtpSender maintains a cache of RTP parameters on the signaling
thread to improve performance and avoid frequent blocking calls to the
worker thread. However, some parameter changes—such as encoder
fallbacks or autonomous codec switches—occur directly on the worker
thread.

These out-of-band updates previously left the signaling thread cache
in a stale state, leading to consistency check failures and potential
logic errors.

This change introduces a callback mechanism in the MediaSendChannel to
notify the RtpSender when parameters change autonomously. Upon
receiving this notification, the RtpSender invalidates its cached
parameters and transaction ID on the signaling thread, ensuring
subsequent requests fetch fresh data from the worker thread.

Additionally, this re-enables the consistency RTC_DCHECK in
GetParametersInternal that was previously disabled due to these cache
mismatches.

Bug: b/478050997
Change-Id: Icd57e04670d7af33844db4d58685c02314a2f531
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/452480
Reviewed-by: Sameer Vijaykar <samvi@google.com>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47036}