Answer Node not work in iteration (Also missing "message" event in SSE response )

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### Dify version

1.16.0-rc1

### Cloud or Self Hosted

Self Hosted (Docker)

### Steps to reproduce

### ✔️ Expected Behavior

The answer node within the iteration outputs correctly.

in 1.14.2, it works just fine, and /chat-messages api received "message" event in streaming mode, after upgrade to 1.16.0-rc1, only "message_end" event received which related to message

### ❌ Actual Behavior

look like the flow just ignored the answer node

and "message" event is missing(see my chatflow events which has answer node in iteration)

Start at the /chat-messages streaming API and the workflow path for an Answer node inside an iteration; compare 1.14.2 with 1.16.0-rc1 to trace why only message_end is emitted. Done means the Answer node produces output and the streaming response includes the expected message event.

Written by the indexing model from the issue text.