fix: preserve original hostname on HTTP socket when using DNS interceptor

[mcollina]

Fixes

#5573

Problem

When the DNS interceptor resolves a hostname and replaces the connection origin with the resolved IP address, the original hostname is lost for HTTP connections. For HTTPS, the hostname remains accessible through TLS SNI (servername), but for plain HTTP it is nowhere available on the socket.

This breaks diagnostics channel instrumentation that relies on the original hostname for metrics attribution.

Changes

1. lib/dispatcher/client.js: Sets client[kServerName] for HTTP connections when request.servername differs (the DNS interceptor already sets servername in dispatch opts)

2. lib/core/connect.js: Overrides socket._host with the original hostname for HTTP connections when servername is provided

3. test/interceptors/dns.js: Adds test verifying socket._host is the original hostname when using the DNS interceptor with HTTP

Verification

$ node -e "...
const { Agent, fetch, interceptors } = require('.');
const { dns } = interceptors;
...
const dispatcher = new Agent().compose(dns({
  lookup: (_origin, _opts, cb) => {
    cb(null, [{ address: '127.0.0.1', family: 4, ttl: 60_000 }]);
  }
}));
const response = await fetch('http://localhost:PORT', { dispatcher });
// socket._host is 'localhost', not '127.0.0.1'

All existing DNS interceptor tests pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.46%. Comparing base (197a83d) to head (f89935a).
⚠️ Report is 25 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5581      +/-   ##
==========================================
+ Coverage   93.44%   93.46%   +0.02%     
==========================================
  Files         110      110              
  Lines       37443    37601     +158     
==========================================
+ Hits        34987    35144     +157     
- Misses       2456     2457       +1     

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