Assign Variable Node Inside Loop Cannot Be Tested Separately & Gets Skipped During Execution (v1.14.2–1.15.0)

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### Dify version

1.15.0

### Cloud or Self Hosted

Self Hosted (Docker)

### Steps to reproduce

Add an Assign Variable node inside the loop node. This node cannot be individually run for testing (no triangular execute button). The Assign Variable node is automatically skipped when the loop is executed. This issue exists in the latest versions from 1.14.2 to 1.15.0, and it can be reproduced both in local deployments and the official Chatflow platform.

### ✔️ Expected Behavior

After adding an Assign Variable node inside a loop node:
1. The Assign Variable node should display a triangular run button to support independent single-node testing.
2. The Assign Variable node should execute normally without being skipped when the loop is running.

### ❌ Actual Behavior

After adding an Assign Variable node inside a loop node:
1. The Assign Variable node has no triangular run button and cannot be tested alone.
2. The node is automatically skipped during loop execution.
This bug occurs across all versions from 1.14.2 to 1.15.0, and can be reproduced on both local deployment environments and the official Chatflow platform.