fix: workflow incorrectly marked as completed while nodes are still executing

[Chenyl-Sai]

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Fixes #30195
Fixes #29316

Summary

When the last upstream node is skipped, the downstream node is added only to _ready_queue and not to _executing_nodes.
After the worker thread consumes _ready_queue, both _ready_queue and _executing_nodes become empty.
As a result, the _dispatcher_loop exits the while loop incorrectly.

Can reproduct use thie DSL: CheckSkip.yml

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[gemini-code-assist]

Summary of Changes

Hello @Chenyl-Sai, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical bug in the workflow engine that caused workflows to be prematurely terminated. The issue stemmed from an oversight in how downstream nodes were tracked when an upstream node was skipped; they were enqueued but not registered as actively executing. This led the workflow dispatcher to incorrectly perceive the workflow as complete. The implemented change ensures that all enqueued nodes are properly registered as executing, thereby maintaining accurate workflow state and preventing early termination.

Highlights

• Addresses Premature Workflow Completion: This pull request fixes a bug where workflows were incorrectly marked as completed while certain nodes were still pending execution, specifically when an upstream node was skipped.
• Corrects Node Execution State: The core issue was that downstream nodes, when enqueued after an upstream skip, were not being added to the _executing_nodes list, leading the dispatcher loop to exit prematurely.
• Ensures Proper Node Tracking: The fix ensures that when a downstream node is enqueued, it is also explicitly marked as 'executing' by calling _state_manager.start_execution, preventing the workflow from terminating early.

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[gemini-code-assist]

Code Review

This pull request correctly identifies and fixes a race condition where a workflow could be marked as complete while nodes were still executing. The fix involves ensuring that when a node is enqueued due to a skip propagation, it is also correctly marked as executing. However, the current implementation could lead to a node being enqueued multiple times. I've provided a critical suggestion to prevent this potential issue by adding a state check before enqueueing the node.

[crazywoola]

Please add a test for this fix as well.