fix: add signalingState guard in ICE restart handler (WT-986)

[SERDUN]

Summary

• Wraps the ICE restart block with a double `signalingState` check to prevent a crash when a remote offer arrives mid-flight
• Outer guard prevents starting a conflicting restart if the peer connection is not in stable state
• Inner guard prevents calling `setLocalDescription` (and sending the `UpdateRequest`) if state changed during the async `restartIce()` + `createOffer()` gap
• Adds warning logs when ICE restart is skipped due to non-stable signaling state
• Includes `call_state_test.dart` — unit tests for `ActiveCall`, `CallState`, transfer state machine, audio device helpers, `CallServiceState`, and `JsepValue` SDP parsing (added as a separate concern to fix analyzer warnings that blocked the push)

Test plan

• Verify ICE restart still works on a normal call reconnect
• Verify no crash occurs when a remote offer arrives while ICE restart is in progress

[SERDUN]

Closing in favour of a consolidated PR that includes the ICE restart guard, renegotiation guard, and glare condition fix together.