Show actual incorrect constant rather than "const must be equal to constant"

I know we are using the raw message and just passing it along:

```
engines: {
type: 'object',
properties: {
node: {
const: '>=18.12.0',
},
},
required: ['node'],
},
```

```
33 | "engines": {
> 34 | "node": ">=18.12.1"
| ^^^^^^^^^^^ 👈🏽 const must be equal to constant
35 | },
```

but I wonder if we could do better. We already know the constant value from the schema, could we not include that in the error message? Or is that outside the scope of this project?

Start by tracing how the raw AJV const-validation message is passed through the formatter for the schema and output shown here. Done means the rendered error identifies the expected constant instead of only saying “const must be equal to constant”; locate the relevant existing coverage and update it for this example.

Written by the indexing model from the issue text.