fix(nuxt): support Nuxt 5

[onmax]

🔗 Linked issue

N/A — this is a compatibility fix for Nuxt 5 consumers.

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This declares Nuxt 5 compatibility and makes GraphQL Vite plugin registration idempotent by plugin name.

Nuxt can extend separate client and SSR Vite configs over a shared plugin array, and another module or the consumer may already provide @rollup/plugin-graphql. Appending another graphql plugin makes the second transform parse the JavaScript emitted by the first as GraphQL. The module now reuses one plugin instance and only adds it when no graphql plugin is already registered.

The playground preloads the real GraphQL plugin as regression coverage. Before the name-based guard, pnpm dev:build fails on playground/queries/launches.gql with Syntax Error: Unexpected Name "var"; with the guard, both client and SSR builds pass.

✅ Checks

• pnpm lint
• pnpm build
• pnpm dev:build (Nuxt 4.0.2 / Nitro 2.12.4)
• Packed consumer build with nuxt-nightly@5.0.0-29762631.396a4ae3, @nuxt/kit-nightly@5.0.0-29762631.396a4ae3, Nitro 3.0.260610-beta, Vite 8.2.1, a real .gql import, and an existing graphql plugin

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