fix: workflow agent node citations and attributions

[mnasrautinno]

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Summary

Context: Agent node supported citations and attributions with this PR #18558 but during the refactoring and decoupling of nodes and node data PR #22581, the logic to emit retriever resource event was removed by an oversight.
Solution: Added the retriever resource transform logic in the agent node

Screenshots

Before	After

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 @mnasrautinno, 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 addresses a regression in the agent node's functionality, specifically restoring the ability to emit retriever resource events. This ensures that citations and attributions, which were previously supported, are correctly processed and made available, rectifying an accidental removal during a recent refactoring effort. The change ensures the agent node properly communicates resource information.

Highlights

• Restored Retriever Resource Event Emission: Re-implemented the logic within the agent node to emit RunRetrieverResourceEvent for RETRIEVER_RESOURCES message types, addressing an oversight from a previous refactoring that inadvertently removed this functionality.
• Enhanced Agent Node Data Transformation: The _transform_message function in the agent node now correctly handles and processes retriever resource messages, ensuring that relevant citation and attribution data is properly transformed and emitted as events.

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[gemini-code-assist]

Code Review

This pull request addresses a bug where citations and attributions were not functioning correctly in the agent node. The issue stemmed from the accidental removal of logic to emit a retriever resource event during a previous refactoring. The solution reintroduces this logic, ensuring that RETRIEVER_RESOURCES messages are properly handled and the corresponding event is yielded. The fix is correct and well-implemented, though I've pointed out a minor formatting issue.

[autofix-ci]

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