fix(dns): preserve servername on dual-stack fallback

[arshsmith1]

This relates to...

The dns interceptor's dual-stack fallback path.

Rationale

When the dns interceptor resolves a hostname it dispatches to the IP literal but sets servername to the original hostname so TLS checks the certificate against the hostname, not the IP. The dual-stack fallback in DNSDispatchHandler.onResponseError (the ETIMEDOUT/ECONNREFUSED retry to the other address family) rebuilds the dispatch options from this.#opts, a copy of the original options that never carried servername (it was only added to the separate first-try object). The retried HTTPS request then has no servername and an IP-literal origin, so buildConnector falls back to getServerName(<ip>), which returns '', and Node verifies the certificate against the IP literal instead of the hostname. The initial attempt and the fallback end up pinned to different TLS identities for the same logical origin.

Changes

Set servername to the original hostname on the fallback dispatch, mirroring the initial dispatch, and add a regression test asserting both attempts carry the hostname servername.

Features

N/A

Bug Fixes

The dns interceptor keeps the hostname servername on the dual-stack fallback, so certificate verification stays pinned to the hostname instead of the resolved IP literal.

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.57%. Comparing base (a65ec32) to head (94a47e9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6016   +/-   ##
=======================================
  Coverage   91.57%   91.57%           
=======================================
  Files         113      113           
  Lines       43679    43680    +1     
=======================================
+ Hits        39999    40000    +1     
  Misses       3680     3680           

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