Replace task vectors with ScopedOperationsBatcher

Migrate task management in PeerConnection, RTCStatsCollector, and
SdpOfferAnswerHandler from std::vector to ScopedOperationsBatcher. This
simplifies collection and execution of cross-thread operations during
shutdown and teardown.

Key modifications include:
- Updating CloseOnNetworkThread to return a BatchTaskWithFinalizer,
  allowing for coordinated cleanup between the network and signaling
  threads.
- Refactoring the signatures of GetMediaChannelTeardownTasks and
  CancelPendingRequestAndGetShutdownTasks to use batcher references.
- Removing rigid thread block count assertions in the PeerConnection
  destructor to allow for more flexible task yielding.
- Simplifying task execution logic in unit tests.

Bug: webrtc:42222804
Change-Id: Ifdcfba5f0bf9680eb78bd3e8b4888e71b873e6ff
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/461641
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47334}