Feat: Limit the iteration start node to only be the source node #4242 (#4260)

### What problem does this PR solve?

Feat: Limit the iteration start node to only be the source node #4242

### Type of change


- [x] New Feature (non-breaking change which adds functionality)

web/src/pages/flow/canvas/node/iteration-node.tsx

@@ -109,6 +109,7 @@ export function IterationStartNode({
         isConnectable={isConnectable}
         className={styles.handle}
         style={RightHandleStyle}
+        isConnectableEnd={false}
       ></Handle>
       <div>
         <ListRestart className="size-7" />