feat(sveltekit): Detect SvelteKit 3 native tracing from the Vite plugin

[chargome]

As of SvelteKit 3, native server-side tracing is no longer configured in svelte.config.js — it's passed to the sveltekit() Vite plugin instead. Our auto-instrumentation plugin derived onlyInstrumentClient from svelte.config.js, so under Kit 3 it could no longer tell that native tracing was enabled and kept injecting its own server-side load instrumentation. The result was duplicate spans (the SDK's function.sveltekit.server.load stacked on top of Kit's native sveltekit.load).

This PR now reads the resolved SvelteKit config that the sveltekit() plugin exposes via its plugin api.options and treats it as the source of truth for Kit 3.
When native server tracing is enabled there, we skip our own server-load instrumentation. On older SvelteKit versions that don't expose the config this way, detection falls back to the existing svelte.config.js-derived value — so nothing changes for Kit 2.

ref #21502

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[chargome]

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