fix: don't mask errors with a "header is already set" error when rendering the error page

[Nic-Polumeyv]

If the root layout load calls setHeaders and something throws while the page renders, the error page dies too. respond_with_error re-runs the root layout load on the same event, so the second setHeaders call hits the set-once check and throws "cache-control" header is already set. That error replaces the real one in handle_error_and_jsonify, so the log shows only the setHeaders complaint and the response falls back to the static error page instead of +error.svelte.

The set-once throw is by design (#5778), so it stays for normal rendering. The fix skips duplicate keys only while the error page renders (state.error), keeping the value from the original render. Same load, same request, so the values are identical anyway. #14681 already carved server-timing out of this throw.

load errors take the nearest-boundary path without re-running the root layout, so the test throws during SSR render instead. The basics root layout sets the header only for the test route (existing locals.url pattern), keeping every other test response unchanged. Fails on main, passes with the fix, in both dev and build.

The server-timing append would double up during the re-run. That predates this change, so I left it.

Fixes #11676

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 5767856:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/5767856a7c6f50253ce3c79fe355e0c17fdfd639

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16323

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Latest commit: 5767856

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