Redesign: Implement RTX PT convention and enhance video codec support

- Implemented conventional payload type assignment for RTX (Primary PT
  + 1) in CodecVendor.
- Enabled FlexFEC PT assignment by checking both WebRTC-FlexFEC-03 and
  WebRTC-FlexFEC-03-Advertised field trials.
- Restored strict error handling in CodecVendor for invalid MID recycling
  (media type mismatch).
- Expanded CodecVendorRedesignTest with coverage for MID recycling failure,
  FEC PT assignment, and RTX convention.
- Improved FakePayloadTypeSuggester conflict detection and fixed
  PayloadTypeRecorder mapping retrieval.
- Updated redesign status and clarified MID recycling constraints in g3doc.

Bug: webrtc:360058654
Change-Id: I1f139e4e82094cc16db00ed54b1d682567e24e61
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/473660
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47741}