feat: support external turnkey dev servers

[brenelz]

Summary

• let provider-owned and non-runnable SSR environments serve the generated turnkey request handler in development
• transport entry CSS through an HMR-tracked virtual module and compose server functions in the external handler
• add ssr.external for integrations that own server build wiring, plus turnkey coverage for the external flow

Verification

• pnpm build
• pnpm check
• pnpm --filter example-turnkey test -- external (150/150 assertions)

[changeset-bot]

🦋 Changeset detected

Latest commit: 908e717

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
vite-plugin-solid	Patch

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[pkg-pr-new]

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npm i https://pkg.pr.new/vite-plugin-solid@286

commit: 908e717