[Agent] Respond node can't get output from agent node with Function Calling policy

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### Dify version

v1.15.0

### Cloud or Self Hosted

Self Hosted (Docker)

### Steps to reproduce

As shown figure, just configure the agent node with func call, but the response node can't get output from the agent

- Function Calling with Thinking mode on

But the weird case is, if I disable the thinking mode, the response node can get output from the agent

- Function Calling with Thinking mode off

Another bug is, the variable from agent was duplicated

### ✔️ Expected Behavior

Get response

### ❌ Actual Behavior

_No response_

Reproduce the issue on Dify v1.15.0 in a self-hosted Docker setup with an Agent node using Function Calling and Thinking mode enabled. Compare the response-node output and agent variable values with Thinking mode disabled; done means the response is returned and the agent variable is no longer duplicated.

Written by the indexing model from the issue text.