Use callbacks and RunLoop in PeerConnection ICE tests

Replace polling-based synchronization with an event-driven approach
using callbacks and test::RunLoop in PeerConnection ICE unit tests.
This change improves test reliability and accuracy by avoiding fixed
poll intervals and explicitly managing thread roles.

* Update PeerConnectionWrapper to include WaitForIceGatheringDone,
  which leverages observer callbacks instead of WaitUntil polling.
* Enhance MockPeerConnectionObserver and CreateSessionDescription
  observers to support quit closures and event-specific callbacks.
* Transition from AutoSocketServerThread to explicit network and
  worker threads in ICE tests to better simulate asynchronous
  operations.
* Replace manual message processing and WaitUntil calls with RunLoop
  execution controlled by event triggers.
* Ensure network-related configuration calls are executed on the
  correct thread using BlockingCall to maintain thread safety.

Bug: none
Change-Id: I4ae0b953f536f780c6cfd39e4ccc3907406c2aec
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/447260
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46851}