Cost validator throws duplicate error for an invalid input

Discussed in https://github.com/mirumee/ariadne/discussions/934

<div type='discussions-op-text'>

^{Originally posted by jayeshv September 30, 2022} Consider the following app

from ariadne import QueryType, ScalarType, gql, make_executable_schema
from ariadne.asgi import GraphQL
from ariadne.validation import cost_directive, cost_validator

type_defs = gql("""
    scalar MyScalar

    type Query {
      test(myInput: MyScalar!): String
    }
""")

query = QueryType()
my_scalar = ScalarType("MyScalar")

@my_scalar.value_parser
def validate_myscalar(value):
    if value == "a":
        raise ValueError("Invalid myscalar")
    return value


@query.field("test")
def resolve_hello(_, info, **kw):
    return "hello"


schema = make_executable_schema([type_defs, cost_directive], query, my_scalar)
app = GraphQL(schema, debug=False, validation_rules=[cost_validator(maximum_cost=5)])

for the following query, it generates two error objects.

query test {
  test(myInput: "a") 
}

{
  "error": {
    "errors": [
      {
        "message": "Argument 'myInput' has invalid value \"a\".\n\nGraphQL request:2:17\n1 | query test {\n2 |   test(myInput: \"a\")\n  |                 ^\n3 | }"
      },
      {
        "message": "Expected value of type 'MyScalar!', found \"a\"; Invalid myscalar",
        "locations": [
          {
            "line": 2,
            "column": 17
          }
        ]
      }
    ]
  }
}

Isn’t the first error created by the cost_validator a redundant one? This duplicate error happens only with the extra validator. ariadne version 0.16.1</div>

[rafalp]

Query cost validator requires extra setup to support variables, its documented here:

https://ariadnegraphql.org/docs/query-validators#exposing-query-variables-to-cost_validator

I guess we should update the docs to make validators setup dynamic mandatory for Query Cost.

[rafalp]

Punting this from 0.17, I don’t feel this is big issue that should prevent the release.