Expand Windows DNS sync-query detection for inline-completing names

[Copilot]

This broadens the Windows DnsResolver workaround for pre-22000 DnsQueryEx inline completion so it covers the full set of names the OS team identified as synchronously completing, not just localhost. The change keeps the existing async path for all other names and avoids the extra DNS_QUERY_NO_WIRE_QUERY probe.

• Behavior

  • Route these names directly through the synchronous path on affected Windows versions:
    • IP literals
    • machine hostname
    • loopback
    • localhost
    • ..DnsServers
    • ..localmachine

• Implementation

  • Replace the localhost-only check with a broader predicate for synchronously completing names.
  • Use IPAddress.IsValid(...) for allocation-free IP literal detection.
  • Compare against the current hostname without process-wide caching.
  • If hostname lookup fails, fall back to the normal async path rather than introducing a new failure mode.

• Regression coverage

  • Expand the existing Windows regression test to cover the added synchronous-completion cases through both sync and async entry points.

if (IPAddress.IsValid(name))
{
    return true;
}

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[Copilot]

Pull request overview

This PR updates the Windows System.Net.NameResolution DnsQueryEx workaround by expanding the set of query names treated as “known to complete synchronously” on affected Windows versions, and extends the existing regression test to exercise those names through both sync/async resolver entry points.

Changes:

• Replace the localhost-only fast-path with IsSynchronouslyCompletingQueryName, including IP literal detection and additional special-case names.
• Route those names directly through the synchronous DnsQueryEx path when running async on pre-22000 Windows versions.
• Expand the Windows regression test to cover the additional “synchronously completing” names.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/System.Net.NameResolution/src/System/Net/DnsResolverPal.Windows.cs	Broaden the pre-22000 async inline-completion workaround to a wider set of names via a new predicate.
src/libraries/System.Net.NameResolution/tests/FunctionalTests/DnsResolverTest.cs	Expand the existing Windows regression test to cover the broader set of synchronously-completing names.

[MichalPetryka]

Should we add an assert that we never got null and a memory leak here?

[rzikm]

/ba-g System.Text.Json failures are unrelated