Review and document path generation in collect_all_errors feature

[Copilot]

Reviewed path generation for schema_path and instance_path in the collect_all_errors feature per maintainer request. Implementation is correct with intentional design choices.

Changes

• Resolved FIXME for UTF-8 validation: UTF-8 length check errors now report the relevant schema keyword (/minLength, /maxLength, or /pattern) instead of empty schema_path
• Clarified indexing pattern: Added comments explaining why internal schema resolution uses i-1 (0-based JSON Pointer format) while error paths use i (1-based Lua indexing)
• Documented design decision: Added API documentation note that paths use 1-based indexing to match Lua conventions, not RFC 6901's 0-based format
• Removed misleading TODO: Clarified that empty instance_path for additionalProperties errors is intentional (refers to the object, not a specific property)

Path Format

-- Paths use 1-based indexing for Lua developers
{
  schema_path = "/allOf/1/required",    -- 1-based, not 0-based
  instance_path = "/user/emails/2",     -- 1-based array index
  error = "property name is required"
}

This deviates from JSON Pointer (RFC 6901) but aligns with Lua's native arrays, improving error message clarity for the target audience.

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