✨(backend) add metadata collection of VAD, connection and chat events

Introduce MetadataCollector and MetadataCollectorService classes to
centralize the collection and storage of user connections, VAD events,
and chat messages. This creates a structured foundation for future speaker
assignment logic based on voice activity detection. Add tests for this new
feature.

src/agents/exceptions.py

@@ -0,0 +1,5 @@
+"""Storage parsers specific exceptions."""
+
+
+class MissingConfigError(Exception):
+    """Raised when a variable is not set in configuration."""