Show the discovered actor address

Print the discovered service's remote channel address instead of
the default `Portal` object repr.

Review: https://github.com/goodboy/tractor/pull/512#discussion_r3883250916

(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))

examples/service_discovery.py

@@ -22,7 +22,8 @@ async def main(service_name: str) -> None:
         async with tractor.wait_for_actor(
             service_name,
         ) as actor_portal:
-            print(f'my_service is found at {actor_portal}')
+            service_addr = actor_portal.chan.raddr
+            print(f'my_service is found at {service_addr}')
 
         await an.cancel()