Enforce thread safety and improve immutability in CodecVendor

This change improves the thread safety of `CodecVendor` by adding a
sequence checker to ensure methods are invoked on the correct thread or
task queue. It also transitions audio codec members to be const,
preventing modification after initialization.

Key changes include:
* Fixed unit tests to configure the media engine or recreate
    the vendor rather than mutating existing vendor instances.
    This unblocks fixing const design problems in CodecVendor.
* Added `SequenceChecker` to `CodecVendor` and guarded usage of
    video codec members.
* Made `audio_send_codecs_` and `audio_recv_codecs_` const,
    initialized via a new helper function.
* Removed test-only setter methods (`set_audio_codecs`,
    `set_video_codecs`) to enforce immutability.
* Updated `TypedCodecVendor` to require a valid media engine in
    its non-default constructor and refactor the construction of
    `codecs_`.

Bug: webrtc:360058654
Change-Id: Ia22a7384450b98f3adfac200f6bcebb9019fa5a3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/426000
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46227}