Fix duplicate custom node installation in workflow import

[NeuralFault]

When importing workflows with custom nodes that appear multiple times in the node index (e.g., ComfyUI-Impact-Pack), the same extension was being added to MissingNodes multiple times. This caused duplicate InstallExtensionStep objects to be created, leading to multiple git clone attempts for the same extension. The second clone would fail because the directory already exists.

Added deduplication using a HashSet to track which extension titles have already been added to MissingNodes, ensuring each extension is only installed once.

[gemini-code-assist]

Summary of Changes

Hello @NeuralFault, 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 critical bug in the workflow import process that caused redundant and failing attempts to install custom node extensions. By introducing a deduplication step, it ensures that each necessary custom node is identified and processed only once, significantly improving the robustness and reliability of workflow imports, especially for complex setups with frequently appearing nodes.

Highlights

• Duplicate Custom Node Installation Fix: Resolved an issue where importing workflows with custom nodes that appeared multiple times in the node index would cause duplicate installation attempts for the same extension.
• Deduplication Mechanism: Implemented a deduplication mechanism using a HashSet to track which extension titles have already been added to the list of missing nodes, ensuring each extension is only processed once.
• Prevented Installation Failures: This change prevents multiple git clone attempts for the same extension, which previously led to failures because the target directory already existed after the first successful clone.

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[gemini-code-assist]

Code Review

This pull request correctly fixes a bug where duplicate custom nodes were being added for installation when importing a workflow. The use of a HashSet to track added nodes is a good approach. I've added one suggestion to slightly optimize the condition for checking duplicates to avoid unnecessary dictionary lookups.