fix: carry the tracing span in the request store instead of copying the event

[Nic-Polumeyv]

Fixes #15040. Every server-side copy of the request event existed only to vary tracing.current per span. It's now a getter reading getActiveSpan(), the shape floated in #13899 (comment), and the copies are gone. Inside a user-created span, current returns that span rather than the enclosing SvelteKit one.

The data-request copy is gone too; the pathname is now normalized in place instead.

Part of #16519.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 080c26a:

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Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16600

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