fix: conversation variable may not change in the answer node

[hjlarry]

Checklist:

Important

Please review the checklist below before submitting your pull request.

• Please open an issue before creating a PR or link to an existing issue
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

Description

Reproduce

1. create a chatflow with three nodes: start, variable assigner, answer
2. create a conversation variable, in the variable assigner node change the variable, in the answer node output the variable
3. in the debug and preview panel , will always output the previous variable value.

reproduce DSL: test.zip

Type of Change

• 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)
• This change requires a documentation update, included: Dify Document
• Improvement, including but not limited to code refactoring, performance optimization, and UI/UX improvement
• Dependency upgrade

Testing Instructions

test locally

• Test the reproduce DSL above
• Test a variable assigner node and a LLM node, and combine the results in the answer node

[hjlarry]

cc @crazywoola @laipz8200