feat(dsl): add partial Dify workflow node import support

[41tair]

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Related Issue

#154

Summary

• Added DSL import/load support for category 1 2 workflow nodes: http-request, variable-aggregator, assigner, list-operator, question-classifier,
  and parameter-extractor.
• Added app-level runtime variable handling for Dify environment_variables and conversation_variables.
• Preserved structured Slim error metadata and added token-count timeout fallback for Slim-backed LLM nodes.
• Kept container/category 3 nodes unsupported until nested graph execution semantics are defined.
• Added coverage for import validation, node factory behavior, Slim error handling, and real debug YAML execution paths.

Checklist

• This pull request links the issue it resolves or advances
• This pull request title follows Conventional Commits, and any breaking change is marked with !
• If CLA Assistant prompted me, I signed CLA.md in the pull request conversation