Proxy handler breaks compressed analytics payloads

[zizzfizzix]

🐛 The bug

In src/runtime/server/proxy-handler.ts, proxied request bodies are always read and may be transformed, even for compressed binary payloads. This can corrupt or stall analytics requests (e.g. PostHog by default uses gzip-js with compression=gzip-js), leading to repeated proxy timeouts (504 Upstream timeout).

Relevant code (current main)

From src/runtime/server/proxy-handler.ts:

• Body is always read for write methods:
  • if (method === 'POST' || method === 'PUT' || method === 'PATCH') (~208)
  • rawBody = await readBody(event) (~209)
• Privacy branch transforms based on parsed/string body:
  • if (anyPrivacy && rawBody) (~211)
• Forwarded request body is rebuilt:
  • body: body ? (typeof body === 'string' ? body : JSON.stringify(body)) : undefined (~292)
• Timeout is hardcoded:
  • const timeoutId = setTimeout(() => controller.abort(), 15000) (~285)
• On fetch error:
  • returns 204 only for /collect, /tr, /events (~303)
  • otherwise throws message: 'Failed to reach upstream' (~312)

🛠️ To reproduce

https://stackblitz.com/edit/nuxt-starter-bcwumula?file=nuxt.config.ts

🌈 Expected behavior

Proxy should preserve raw request bytes for endpoints/content-types that are not explicitly safe to parse and rewrite, and only apply body privacy transforms where parsing is guaranteed valid.

ℹ️ Additional context

No response