fix: Guard getReader function for other fetch implementations

[chargome]

fixes #13220

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser	22.46 KB (0%)
@sentry/browser (incl. Tracing)	34.24 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay)	70.3 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	63.63 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas)	74.69 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback)	87.3 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback, metrics)	89.15 KB (+0.01% 🔺)
@sentry/browser (incl. metrics)	26.76 KB (0%)
@sentry/browser (incl. Feedback)	39.4 KB (0%)
@sentry/browser (incl. sendFeedback)	27.08 KB (0%)
@sentry/browser (incl. FeedbackAsync)	31.73 KB (0%)
@sentry/react	25.22 KB (0%)
@sentry/react (incl. Tracing)	37.23 KB (+0.01% 🔺)
@sentry/vue	26.6 KB (0%)
@sentry/vue (incl. Tracing)	36.08 KB (+0.01% 🔺)
@sentry/svelte	22.59 KB (0%)
CDN Bundle	23.68 KB (0%)
CDN Bundle (incl. Tracing)	35.92 KB (+0.01% 🔺)
CDN Bundle (incl. Tracing, Replay)	70.35 KB (+0.01% 🔺)
CDN Bundle (incl. Tracing, Replay, Feedback)	75.6 KB (+0.01% 🔺)
CDN Bundle - uncompressed	69.44 KB (0%)
CDN Bundle (incl. Tracing) - uncompressed	106.38 KB (+0.02% 🔺)
CDN Bundle (incl. Tracing, Replay) - uncompressed	218.23 KB (+0.01% 🔺)
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	231.12 KB (+0.01% 🔺)
@sentry/nextjs (client)	37.09 KB (+0.01% 🔺)
@sentry/sveltekit (client)	34.81 KB (+0.01% 🔺)
@sentry/node	114.77 KB (0%)
@sentry/node - without tracing	89.33 KB (-0.01% 🔽)
@sentry/aws-serverless	98.5 KB (+0.01% 🔺)