Make CommonService.transport the canonical way to retrieve transport

The classes internally are set up to allow empty or null transport, which we
almost never want to actually use. Use this property as a way of retrieving
the concrete transport (or failing) rather than handling None logic everywhere
that we don't need it

Author: ndevenish

src/workflows/services/common_service.py

@@ -17,6 +17,7 @@
 
 import workflows
 import workflows.logging
+from workflows.transport.common_transport import CommonTransport
 from workflows.transport.middleware.otel_tracing import OTELTracingMiddleware
 
 
@@ -171,11 +172,14 @@ def config(self):
         return self._environment.get("config")
 
     @property
-    def transport(self):
+    def transport(self) -> CommonTransport:
+        # Handle the fact that we apparently allow missing transport layers
+        if self._transport is None:
+            raise RuntimeError("Transport layer has not yet been defined")
         return self._transport
 
     @transport.setter
-    def transport(self, value):
+    def transport(self, value: CommonTransport) -> None:
         if self._transport:
             raise AttributeError("Transport already defined")
         self._transport = value