refactor: refactor workflow context

[fatelei]

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Summary

resolve #30595

• not use flask context directly in workflow
• use di to pass the flask context
• use register model

add method register_context_capturer to register context, later call capture_current_context to fetch the context, register_context_capturer can register many context, so workflow can use different context, to decouple the flask context, the workflow can not see the flask context.

# Usage Flow

# 1. Flask app initialization (app_factory.py)
init_flask_context()  # Registers Flask capturer

# 2. Workflow execution (framework-agnostic)
from core.context import capture_current_context
ctx = capture_current_context()  # Uses registered capturer

# This achieves true architectural decoupling - the workflow layer is completely unaware of Flask's existence.

Screenshots

Before	After
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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 make lint and make type-check (backend) and cd web && npx lint-staged (frontend) to appease the lint gods

[gemini-code-assist]

Summary of Changes

Hello @fatelei, 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 significantly refactors the workflow context management by introducing an abstraction layer to decouple workflow execution from direct Flask context dependencies. This change aims to enhance the system's robustness, particularly in multi-threaded environments, and improve testability by allowing contexts to be explicitly managed and passed through dependency injection. The core idea is to move away from implicit global state to explicit context objects, making the codebase more maintainable and scalable.

Highlights

• Decoupling Flask Context: The workflow execution logic has been refactored to no longer directly depend on Flask's application context. This improves modularity and testability.
• Introduction of Execution Context Abstraction: A new core.workflow.context package has been introduced, providing an abstract AppContext and a concrete ExecutionContext to manage application context, Python contextvars, and user information in a Flask-independent manner.
• Dependency Injection for Context: Instead of implicitly relying on Flask's global context, the new IExecutionContext interface is now explicitly passed to worker threads and iteration nodes, enabling clearer dependency management.
• Database Session Management: The app_generator.py now utilizes session_factory.create_session() for database sessions, further centralizing and abstracting database access.
• Improved User Resolution: The workflow_as_tool module now uses a new helper function _try_resolve_user_from_request to abstract the process of resolving user information from the Flask request context.
• Enhanced Testability: New unit tests have been added for the execution_context and flask_app_context modules, ensuring the robustness and correctness of the new context management system.

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[gemini-code-assist]

Code Review

This pull request introduces a significant and valuable refactoring by abstracting the Flask context dependency within the workflow engine. The new ExecutionContext abstraction, along with its Flask-specific and null implementations, greatly improves the modularity, testability, and portability of the code. The changes are well-structured and the addition of unit tests for the new context management system is commendable.

I've identified a critical issue in _try_resolve_user_from_request that could lead to a RuntimeError, and a medium-severity issue regarding code duplication in FlaskExecutionContext. Addressing these will make the implementation more robust and maintainable.

Additionally, I noticed that app_generator.py still uses the old preserve_flask_contexts for its worker thread. While this is outside the scope of the changed lines in this PR, I recommend creating a follow-up task to refactor it to use the new ExecutionContext for consistency across the codebase.