Feature Request: Add `fieldName` property to validation error objects

[gsriram24]

Summary

When validation fails, the field name is currently embedded within the issueMessage string, making it difficult to programmatically identify which field caused the error without string parsing.

Current Behavior

{
  issueMessage: 'Required attribute "price" is missing',
  severity: 'ERROR',
  path: [{ type: 'Product' }, { property: 'offers' }, { type: 'Offer' }]
}

To extract the field name, consumers must parse the message:

const match = issue.issueMessage.match(/attribute "([^"]+)"/);
const fieldName = match?.[1]; // "price"

Problem

This approach is:

• Fragile — breaks if message wording changes in future versions
• Redundant — every consumer implements the same parsing logic
• Not i18n-friendly — would break if messages are localized

Proposed Solution

Add a fieldName property to error objects returned by required(), recommended(), and or() methods in src/types/base.js:

{
  issueMessage: 'Required attribute "price" is missing',
  severity: 'ERROR',
  path: [{ type: 'Product' }, { property: 'offers' }, { type: 'Offer' }],
  fieldName: 'price'  // New property
}

For or() conditions checking multiple fields:

{
  issueMessage: 'One of the following conditions needs to be met: ...',
  severity: 'ERROR',
  path: [...],
  fieldName: 'price',                              // First field (simple access)
  fieldNames: ['price', 'priceSpecification.price'] // All fields
}

Use Case

This enables precise UI feedback in structured data editors — for example, highlighting the exact form field that needs attention without fragile regex parsing.

Backward Compatibility

This change is fully backward compatible — it only adds new optional properties to existing error objects.

Additional Validators

Beyond base.js, some validators create error objects directly without using the base methods. These should also include fieldName for consistency:

File	Error Context	fieldName
BreadcrumbList.js	At least two ListItems required	itemListElement
BreadcrumbList.js	Invalid URL validation	item or item.@id
DefinedRegion.js	addressRegion/postalCode conflict	addressRegion
MerchantReturnPolicy.js	Return fees validation	(context-dependent)
Product.js	aggregateRating/offers/review required	aggregateRating + fieldNames
Product.js	Notes count warning	review
ProductMerchant.js	GTIN field missing	gtin + fieldNames
Rating.js	Rating outside range	ratingValue
schemaOrg.js	Unsupported property	(property name from context)

This ensures fieldName is consistently available across all validation errors, not just those using base methods.