Set-Cookie header gets concatenated in fetch if an interceptor is in place

[hexchain]

Bug Description

If the dispatcher contains an interceptor, even a dummy one, the Set-Cookie headers will be concatenated together.

Reproducible By

const http = require('node:http')
const {Agent} = require('undici');

const dispatcher = new Agent().compose((dispatch) => dispatch)

const server = http.createServer((req, res) => {
    res.writeHead(200, [['set-cookie', 'a=1'], ['set-cookie', 'b=2'], ['set-cookie', 'c=3']])
    res.end("ok")
})
server.listen({port: 3002}, () => {
    Promise.all([fetch('http://localhost:3002'), fetch('http://localhost:3002', {dispatcher})])
        .then(([r1, r2]) => {
            console.log(r1.headers.getSetCookie())
            console.log(r2.headers.getSetCookie())
        })
        .then(() => server.close())
});

The output will be:

[ 'a=1', 'b=2', 'c=3' ]
[ 'a=1,b=2,c=3' ]

Expected Behavior

The two outputs should be identical.

Environment

Nodejs 24.4.1
Undici 7.13.0

[hexchain]

I took a stab at this by changing WrapHandler. Another possible fix is to modify httpNetworkFetch->dispatch->onHeaders instead. Please let me know if that is preferred.