fix(sveltekit): Avoid invalidating data on route changes in wrapServerLoadWithSentry

[Lms24]

As reported in #8610, our SvelteKit SDK caused server load data to be invalidated, resulting in said functions being called on every route change. I initially thought this was related to our Kit-specific client fetch instrumentation which turned out not to be causing this. Instead, the culprit is our wrapServerLoadWithSentry wrapper:

In the wrapper, we access event.route.id to determine the route of the load function for the span description. Internally, SvelteKit puts a proxy on certain event properties, such as event.route. In case any property of event.route was accessed, SvelteKit marks this internally and send along a flag to the client. On a route change, the client would check this flag and mark the route as invalidated, thereby causing a call to the load function on each navigation.

Took me quite a while to find the cause but thanks to @kamilogorek we found a fix to access the route id without invoking the proxy.

closes #8610

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	75.12 KB (+0.36% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.14 KB (+0.09% 🔺)
@sentry/browser - Webpack (gzipped)	21.81 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	69.72 KB (+0.27% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	28.18 KB (+0.1% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	20.18 KB (+0.05% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	219.89 KB (+0.38% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	84.78 KB (+0.13% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	59.86 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.04 KB (+0.09% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	65.3 KB (+0.39% 🔺)
@sentry/react - Webpack (gzipped)	21.84 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	92.94 KB (+0.32% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	50.66 KB (+0.1% 🔺)