chore: run each platform test variant through one reusable workflow

[Nic-Polumeyv]

The version-3 half of #17171: one reusable workflow called per variant replaces the six per-platform workflows and four actions, with the Bun and Netlify instrumentation tests kept.

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 106b495:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/106b4952f8fc2a75539046f91fd164c645316b0f

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/17173

[changeset-bot]

⚠️ No Changeset found

Latest commit: 106b495

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-17173-svelte.vercel.app/

[teemingc]

I was a bit confused because the changes differ a little from the ones on main. This PR combines more of the steps into a single file.

could we merge #17176 then #17164 before this one?

[Nic-Polumeyv]

This now has the step names from #17176 and your vercel.json removal, so it doesn't need to wait on either. #17164 deletes node-test while platform-tests-bun.yml still uses it and drops the instrumentation and Windows node legs, so I think it makes sense to close it in favor of this one

[teemingc]

#17164 also resolves the merge conflicts between main and version-3

[Nic-Polumeyv]

Could this go in first then? #17164 can keep version-3's side for the platform workflows, so the bun break never lands.