feat: create generic fixedCollection validation utility
- Add FixedCollectionValidator utility to handle all fixedCollection patterns - Support validation for 12 different node types including Switch, If, Filter, Summarize, Compare Datasets, Sort, Aggregate, Set, HTML, HTTP Request, and Airtable - Refactor enhanced-config-validator to use the generic utility - Add comprehensive tests with 19 test cases covering all node types - Maintain backward compatibility with existing validation behavior This prevents the 'propertyValues[itemName] is not iterable' error across all susceptible n8n nodes, not just Switch/If/Filter.
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