fix: reuse cached fetch responses when a cross-origin url is not in canonical form

[Nic-Polumeyv]

closes #14781

During SSR, fetch responses are embedded as <script data-sveltekit-fetched data-url=...> tags, and during hydration the client rebuilds the url to find them instead of hitting the network. The two sides build it differently. The server serializes the normalized href from new URL(input, event.url), while the client keeps the raw string, because prerendered pages may be served from any origin. That reasoning holds for same-origin urls, which are serialized path-relative, but cross-origin urls are absolute on both sides, so a raw string like http://localhost:8080 without the trailing slash misses the cache and the request fires a second time from the browser.

The fix normalizes requested to resolved.href in the cross-origin case only. Normalizing in build_selector instead, as suggested in the issue, would break the same-origin case, new URL('/mock') throws without a base. requested also became a const derived from resolved since the mutation was no longer needed.

The new test fails on main, rendering count: 2 after the hydration refetch overwrites the SSR data.

Not fixed here: request bodies are hashed as strings on the server but dropped from the client hash when they are not strings, so POST-with-body fetches still miss the cache. Can follow up separately.

Caching under the normalized href also meant a non-GET fetch spelled without the trailing slash could no longer evict the entry, so the second commit derives the eviction key the same way.

Also not fixed: on prerendered pages the server classifies same-origin against prerender.origin while the client uses the live origin, so an absolute url pointing at the deploy origin still misses the cache.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 5c0a735:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/5c0a735746fad6936592387352305ae163f19f4a

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16339

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