test: fuzz test schema exec

[pavelnikolov]

Add fuzz testing for schema execution and error early for executable documents without any operations.

[Copilot]

Pull Request Overview

This PR adds fuzz testing for GraphQL schema execution and introduces an early error when an executable document has no operations.

• Adds a guard in ValidateWithVariables to return an error if the parsed document contains zero operations.
• Introduces FuzzSchema_Exec in fuzz_test.go with a set of seed queries and fuzz logic to validate execution behavior.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
graphql.go	Early return of a QueryError when doc.Operations is empty
fuzz_test.go	New fuzz test FuzzSchema_Exec to exercise Exec paths

Comments suppressed due to low confidence (2)

fuzz_test.go:20

• You may want to add a seed input containing only fragment definitions (no operations) to ensure the new early-error path in ValidateWithVariables is exercised by the fuzzer.

queries := []string{

fuzz_test.go:11

• [nitpick] Consider renaming FuzzSchema_Exec to FuzzSchemaExec (dropping the underscore) to match Go fuzz test naming conventions.

func FuzzSchema_Exec(f *testing.F) {