test(nextjs): Add response content length to assertion

[AbhiPrasad]

Seems this sneaked by #8139

Needed so we can release.

[AbhiPrasad]

Closing actually we should make sure this is an integer as per the spec: https://develop.sentry.dev/sdk/performance/span-data-conventions/

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	21.03 KB (+0.06% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	65.68 KB (+0.09% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	19.59 KB (+0.11% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	58.15 KB (+0.12% 🔺)
@sentry/browser - Webpack (gzipped + minified)	21.2 KB (+0.16% 🔺)
@sentry/browser - Webpack (minified)	69.07 KB (+0.05% 🔺)
@sentry/react - Webpack (gzipped + minified)	21.22 KB (+0.16% 🔺)
@sentry/nextjs Client - Webpack (gzipped + minified)	49.15 KB (+0.08% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	28.66 KB (+0.05% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	26.9 KB (+0.08% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	48.19 KB (+1.21% 🔺)
@sentry/replay - Webpack (gzipped + minified)	42.06 KB (+1.45% 🔺)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	67.16 KB (+0.99% 🔺)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	60.06 KB (+1.15% 🔺)