Invalid constants can pass validation

[mark-koch]

The following Hugr validates, even though { "c": "ConstF64", "v": 42 } is an incorrect serialisation of float values (it should be { "c": "ConstF64", "v": { "value": 42.0 } } instead):

{
   "version": "v1",
   "nodes": [
      {
         "parent": 0,
         "input_extensions": null,
         "op": "Const",
         "v": {
            "v": "Extension",
            "extensions": [
               "arithmetic.float.types"
            ],
            "typ": {
               "t": "Opaque",
               "extension": "arithmetic.float.types",
               "id": "float64",
               "args": [],
               "bound": "C"
            },
            "value": {
               "c": "ConstF64",
               "v": 42
            }
         }
      }
   ],
   "edges": [],
   "encoder": null
}

[doug-q]

I think this bug comes from OpaqueValue treating these two cases as the same:

• can't deserialize because it's an unknown CustomConst
• a known CustomConst failed to deserialize

when they should be treated differently. The first case is fine, leave a CustomSerialized in the Hugr. The second case should result in a deserialization failure.

If that is true then this is distinct from #1185, which is about validation not happening at all.

[ss2165]

Remaining parts of this covered by #1225