Fixing play state conflict

[HoMinhNhat123]

Describe your changes

Fixed a bug in playNodesUpTo in FlowProvider.tsx where calling it while playAllNodes was already running would cause nodes to re-execute mid-run concurrently. This is especially harmful for slow nodes — a user repeatedly clicking a node while it is still loading would keep restarting execution, and the node would never finish. The fix adds a guard at the top of playNodesUpTo:

if (playAllStateRef.current != null) return;

Added unit tests for FlowProvider to document and prove existing bugs in 'playNodesUpTo'. The tests cover:

• onConnect correctly initializes merge node source slots when connecting multiple nodes
• applyNewOutput propagates output to correct downstream nodes
• applyNewOutput handles merge flow nodes with multiple inputs
• playAllNodes executes nodes in topological order level by level
• playNodesUpTo correctly guards against running while playAllNodes is in progress (proving the bug where playNodesUpTo had no guard against playAllStateRef, which would cause nodes to re-execute mid-run)

Issue resolved by this PR (if any)

• Issue Number: 127
• Link: Clicking Play on a node during Play All freezes the workflow #127

Type of change (Check all that apply)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update
• Other:

Parts of Curio impacted by this PR:

• Frontend
• Backend
• Sandbox

Testing

• Executed ./scripts/test.sh and it passed all tests.

Backend unit tests, sandbox unit tests, and Jest passed. E2E Playwright tests (06-autark-what-if-shadow-study, 07-autark-gpu-shader) fail locally due to no hardware WebGPU in WSL

Screenshots (if relevant)

Checklist (Check all that apply)

• I have manually loaded each .json test from the tests/ folder into Curio, ran all the nodes one by one, and checked that they run without errors and give the expected results
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules