chore: fix and simplify the platform tests

[Nic-Polumeyv]

Follow-up to #17143. Environment secrets only resolve in a called workflow when the caller passes secrets, so the deploy jobs get empty tokens, and pnpm/setup rejects the pnpm 10 that PRs based on main use.

This passes secrets: inherit, reads the pnpm version from the commit under test, and replaces the five per-platform workflows and four actions with one reusable workflow called per variant. The Vercel deploy now creates the project's root directory before deploying, and the test job gets the build output that the Netlify edge test reads.

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 4a9eb01:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/4a9eb01686f330bb452f0862ee3cd440de1948ad

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/17171

[changeset-bot]

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Latest commit: 4a9eb01

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[teemingc]

Thank you so much