Apply query token limit in Schema.validate

[ydah]

Schema.validate(string) parsed query strings without passing the schema's max_query_string_tokens setting. This allowed validation-only callers to bypass the configured token limit.

LimitedSchema.validate("{ value value }")
# Before => validation proceeds despite exceeding the token limit
# After => raises GraphQL::ParseError

This passes max_query_string_tokens to GraphQL.parse and adds regression coverage.