perf: inline content stream setup

[Phillip9587]

Inline content stream initialization into read to keep the stream and content-length state local. Only read content-length when needed and avoid temporary mutations of req.length and stream.length.

[Copilot]

Pull request overview

This PR refactors request body reading in lib/read.js by inlining the content stream initialization into read, keeping stream and content-length handling local and avoiding temporary mutation of req.length / stream.length.

Changes:

• Inline content-encoding detection and inflate gating directly into read.
• Only read content-length when content-encoding is identity and use req directly as the stream.
• Remove the now-unnecessary private contentstream helper.

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[bjohansebas]

How much of a performance improvement does this provide?