Add Instance Validation Conformance Tests

[Shubh-Raj]

Add Instance Validation Conformance Tests

Description

The validate/ folder currently has the step definitions and npm script (npm run test:instances) wired up, but validate/features/validate.feature is completely empty — there are zero instance validation test scenarios.

Instance validation (validating JSON data against Concerto models at runtime) is a core part of the Concerto runtime. Having conformance tests for this is critical, especially for validating cross-platform runtime implementations like the ongoing Rust port (concerto-rust, concerto-validate-rs).

Current State

• validate/validateSteps.js — Step definitions exist with 3 step types
• npm run test:instances — Wired up in package.json
• validate/features/validate.feature — Empty (0 scenarios)
• validate/models/validate/ — Only 2 sample files (1 valid, 1 invalid)

Proposed Changes

Add ~10 scenarios covering the following instance validation rules:

1. Primitive Type Validation

• Valid instance with all primitive types (String, Integer, Double, Long, Boolean, DateTime) should pass
• Type mismatch (e.g., string value for Integer field) should fail
• Type mismatch (e.g., string value for Boolean field) should fail

2. Required vs Optional Fields

• Missing a required field should fail validation
• Omitting an optional field should pass validation
• All required fields present should pass validation

3. Enum Instance Validation

• Valid enum value should pass
• Invalid enum value (not in enum definition) should fail

4. $class Type Resolution

• $class matching a defined type should pass
• $class referencing a non-existent type should fail

Additional

This extends concerto-conformance to validate the Rust port. Without instance validation conformance tests, there is no way to verify that the Rust runtime correctly validates JSON data against Concerto models.