fix: derive the fetch cache key in one place

[Nic-Polumeyv]

Follow-up to #16339, draft until it merges since the diff includes its commits until then.

The fetch cache key was derived independently in four places (push_fetched, serialize_data, resolve_fetch_url, build_selector) and #14781 was drift between two of them. This derives it in one shared module instead. No change to the serialized format.

This also fixes a bug: the server hashed any truthy request body, so a load fetch with a URLSearchParams or FormData body threw during render. Both sides now use the client's rule and exclude bodies that can't be hashed identically.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from adbc594:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/adbc594804b6a67618e1f005aa1a6785fa1f2f17

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16792

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Latest commit: adbc594

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

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@sveltejs/kit	Patch

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