fix(sveltekit): Handle nested server calls in sentryHandle

[Lms24]

Previously, we always created a new domain when entering the sentryHandle function. This caused "nested" SvelteKit Server calls to create new transaction whenever a new domain was created in the sentryHandle function because the active span is bound to the domain. This PR introduces a check if there is an active transaction, in which case we just execute the handler and don't create a new domain, as we are already in this active domain.

As a result, nested SK server calls become a span of the parent server call instead of a new transaction:

ref #7526

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	20.62 KB (+0.06% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	64.4 KB (+0.02% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	19.16 KB (-0.02% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	56.78 KB (+0.02% 🔺)
@sentry/browser - Webpack (gzipped + minified)	21.53 KB (-0.39% 🔽)
@sentry/browser - Webpack (minified)	72 KB (-0.04% 🔽)
@sentry/react - Webpack (gzipped + minified)	21.55 KB (-0.39% 🔽)
@sentry/nextjs Client - Webpack (gzipped + minified)	51.98 KB (-0.01% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	28.16 KB (+0.06% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	26.36 KB (+0.06% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	44.74 KB (+0.62% 🔺)
@sentry/replay - Webpack (gzipped + minified)	38.86 KB (+0.83% 🔺)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	63.41 KB (+0.46% 🔺)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	56.49 KB (+0.5% 🔺)