Add tests for TracingContext and TurnTraceObserver

Cover pipeline-scoped tracing context lifecycle, span hierarchy,
conversation/turn context management, and concurrent pipeline isolation.

.github/workflows/coverage.yaml

@@ -41,6 +41,7 @@ jobs:
             --extra livekit \
             --extra local-smart-turn-v3 \
             --extra piper \
+            --extra tracing \
             --extra websocket
 
       - name: Run tests with coverage

.github/workflows/tests.yaml

@@ -45,6 +45,7 @@ jobs:
             --extra livekit \
             --extra local-smart-turn-v3 \
             --extra piper \
+            --extra tracing \
             --extra websocket
 
       - name: Test with pytest