Add getting a handle and result of an existing operation

Author: VegetarianOrc

nexus_standalone_operations/README.md

@@ -53,6 +53,7 @@ uv run nexus_standalone_operations/starter.py
 
 ```
 Echo result: hello
+Echo result from existing operation handle: hello
 
 Started `MyNexusService.Hello`. OperationID: hello-...
 `MyNexusService.Hello` result: Hello, World!

nexus_standalone_operations/starter.py

@@ -14,6 +14,7 @@
 
 from nexus_standalone_operations.service import (
     EchoInput,
+    EchoOutput,
     HelloInput,
     MyNexusService,
 )
@@ -33,14 +34,22 @@ async def main() -> None:
     )
 
     # Start sync echo operation and await the result immediately.
+    operation_id = f"echo-{uuid.uuid4()}"
     echo_result = await nexus_client.execute_operation(
         MyNexusService.echo,
         EchoInput(message="hello"),
-        id=f"echo-{uuid.uuid4()}",
+        id=operation_id,
         schedule_to_close_timeout=timedelta(seconds=10),
     )
     print(f"Echo result: {echo_result.message}")
 
+    # Get a handle and the result of an existing operation
+    existing_op_handle = client.get_nexus_operation_handle(
+        operation_id, operation=MyNexusService.echo
+    )
+    existing_result = await existing_op_handle.result()
+    print(f"Echo result from existing operation handle: {existing_result.message}")
+
     # Start async (workflow-backed) hello operation and get a NexusOperationHandle.
     handle = await nexus_client.start_operation(
         MyNexusService.hello,