fix: use content-length header to avoid buffering responses (#711)

[stedfn]

Summary

• requestfinished handler now reads the content-length header first
• Only falls back to res.body() when the header is absent
• Avoids buffering entire response bodies (images, videos, large payloads) just to record byte length

Fixes #711

Test plan

• bun test passes
• bun run build succeeds
• Network buffer still records accurate sizes for responses with content-length header
• Responses without content-length header still get their size recorded via body fallback

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