[Refactor] Extract parseGraphQLDocument helper in bulk operation helpers

[gonzaloriestra]

WHAT is this pull request doing?

Extracts duplicated GraphQL document parsing and syntax error handling from validateSingleOperation and isMutation in packages/cli-kit/src/public/node/api/bulk-operations/helpers.ts into a private parseGraphQLDocument helper function.

How to test your changes?

CI

Checklist

• I have evaluated the impact of my change
• I have added a changeset if this PR contains user-facing changes

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PR created automatically by Jules for task 2009343155212158420 started by @gonzaloriestra

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