Drop wait_for_signal in favor of generic state-poke signal

Express handler-side gating using the existing before_actions +
AwaitWorkflowState primitives. The "unblock" signal on NexusHandlerWorkflow
now just sets workflow_state.kvs["unblocked"]="1", so callers compose
"block then unblock" without a dedicated proto knob.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: stephanos

loadgen/kitchen-sink-gen/src/main.rs

@@ -730,7 +730,6 @@ impl ExecuteNexusOperation {
             before_actions: vec![],
             handler_workflow_id: String::new(),
             handler_workflow_id_conflict_policy: 0,
-            wait_for_signal: false,
         })
     }
 }
@@ -776,7 +775,6 @@ impl<'a> Arbitrary<'a> for ExecuteNexusOperation {
             before_actions,
             handler_workflow_id: String::new(),
             handler_workflow_id_conflict_policy: 0,
-            wait_for_signal: false,
         })
     }
 }