Combine const and enum errors into a single message

[ShivaGupta-14]

Description

When multiple const or enum constraints fail (e.g., via allOf), produce a single message with the most restrictive constraint instead of multiple redundant messages.

• Compute the intersection of allowed values across all constraints
• If intersection has 1 value: use const message
• If intersection has multiple values: use enum message
• If intersection is empty (contradictory): use boolean schema message
• Fixes: Combine const and enum handlers #132

Checklist

• npm test
• npm run lint
• npm run type-check

[jdesrosiers]

This approach isn't going to work. Try,

{
  "allOf": [
    { "const": "a" },
    { "const": "b" }
  ]
}