feat(wfe-core): add step primitives for workflow control flow

12 step primitives implementing StepBody: DecideStep, IfStep, WhileStep,
ForEachStep, SequenceStep, DelayStep, WaitForStep, ScheduleStep,
RecurStep, PollEndpointStep, SagaContainerStep, EndStep.

Each primitive handles its state machine via persistence_data and
branch creation for container steps.

wfe-core/src/primitives/poll_endpoint.rs

@@ -0,0 +1,53 @@
+use async_trait::async_trait;
+
+use crate::models::poll_config::PollEndpointConfig;
+use crate::models::ExecutionResult;
+use crate::traits::step::{StepBody, StepExecutionContext};
+
+/// A step that polls an external HTTP endpoint until a condition is met.
+/// The actual HTTP polling is handled by the executor, not this step.
+pub struct PollEndpointStep {
+    pub config: PollEndpointConfig,
+}
+
+#[async_trait]
+impl StepBody for PollEndpointStep {
+    async fn run(&mut self, _context: &StepExecutionContext<'_>) -> crate::Result<ExecutionResult> {
+        Ok(ExecutionResult::poll_endpoint(self.config.clone()))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::models::poll_config::{HttpMethod, PollCondition};
+    use crate::models::ExecutionPointer;
+    use crate::primitives::test_helpers::*;
+    use std::collections::HashMap;
+    use std::time::Duration;
+
+    #[tokio::test]
+    async fn returns_poll_config() {
+        let config = PollEndpointConfig {
+            url: "https://api.example.com/status".into(),
+            method: HttpMethod::Get,
+            headers: HashMap::new(),
+            body: None,
+            interval: Duration::from_secs(10),
+            timeout: Duration::from_secs(300),
+            condition: PollCondition::StatusCode(200),
+        };
+
+        let mut step = PollEndpointStep {
+            config: config.clone(),
+        };
+        let pointer = ExecutionPointer::new(0);
+        let wf_step = default_step();
+        let workflow = default_workflow();
+        let ctx = make_context(&pointer, &wf_step, &workflow);
+
+        let result = step.run(&ctx).await.unwrap();
+        assert!(!result.proceed);
+        assert_eq!(result.poll_endpoint, Some(config));
+    }
+}