Refactor: Extract Genkit initialization to factory module

[davideast]

This PR extracts the Genkit initialization logic from src/core/dataprompt.ts into a new src/core/genkit-factory.ts file.

This change is part of the "Large Files and Responsibilities" improvement strategy outlined in PREVENT_CONFLICTS.md. By isolating createDefaultGenkit and loadUserGenkitInstance, we reduce the complexity of the main dataprompt entry point and minimize the risk of merge conflicts when modifying initialization logic or adding new plugins.

Changes:

• Created src/core/genkit-factory.ts.
• Moved createDefaultGenkit and loadUserGenkitInstance to the new factory.
• Updated src/core/dataprompt.ts to use the new factory.
• Added tests/unit/genkit-factory.test.ts to verify the extracted logic.

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PR created automatically by Jules for task 6019131064566681597 started by @davideast

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