graphql: full operation-level testing and fuzzing support

[webdevsamran]

Context

GraphQL schemas already load into the normalized contract model and field/type/nullability/enum diffs work.

Work items

• Parse persisted operations (.graphql files) into Operation entries
• Generate schema-driven queries/mutations from the normalized model
• Drift checks against live GraphQL endpoints (introspection-based)
• Error-shape assertions (data/errors envelope validation)

Acceptance criteria

• �piverity test schema.graphql --base-url ... runs positive/negative query cases; drift reports undocumented field usage.