fix(qemu/http): avoid duplicate Content-Length for buffered bodies

[geminixiang]

Fixes #134.

Problem

I found this while running git clone inside a Gondolin VM. On Node.js >= 24.17, guest requests with a buffered body fail with HTTP 502 through the QEMU HTTP bridge. Small POST requests and git clone over smart HTTP are affected, while GET and streamed bodies still work.

I traced the error to content-length. The bridge sets it from the buffered body, then npm undici 6 appends another derived value before dispatching through Node's built-in undici Agent. Built-in undici 7.28 rejects the duplicated value ("181, 181") with invalid content-length header. More details and the Node.js version bisection are in #134.

What I changed

I pass a copy of currentRequest.headers to fetch() and remove content-length from that copy only for buffered bodies, so undici derives it once from the body.

Streamed bodies keep their explicit length. The original currentRequest.headers is not changed, so redirects and response hooks still receive the measured length.

Verification

I verified the change on Node.js 24.18.0 with the QEMU backend:

• A buffered POST returns HTTP 200 instead of 502.
• git clone --depth 1 exits successfully instead of returning 502.
• Raw-wire checks with npm undici 6.23, 6.27, 7.28, and 8.5 produce exactly one Content-Length for buffered and streamed bodies.
• All 97 tests in host/test/qemu-net.test.ts pass on Node.js 24.13 and 24.18.
• TypeScript type checking and Prettier checks pass.

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