[Android] Fix DNS localhost subdomain resolution with IPv6 on Android

[simonrozsival]

Fix DNS localhost subdomain tests with AddressFamily on Android

Fixes #124751

Problem

The DnsGetHostEntry_LocalhostSubdomain_RespectsAddressFamily and DnsGetHostAddresses_LocalhostSubdomain_RespectsAddressFamily tests consistently fail on Android.

The RFC 6761 localhost subdomain fallback (introduced in #123076) resolves "test.localhost" by first trying the OS resolver, then falling back to resolving plain "localhost" with the same AddressFamily. On Android, the second step fails for InterNetworkV6 because Android's default /etc/hosts maps ::1 to ip6-localhost instead of localhost:

127.0.0.1       localhost
::1             ip6-localhost

This causes getaddrinfo("localhost", AF_INET6) to return EAI_NONAME.

Fix

When the localhost fallback fails with InterNetworkV6 on Android, retry the resolution using "ip6-localhost" — Android's default hostname for the IPv6 loopback address.

The fallback chain becomes:

1. getaddrinfo("test.localhost", AF_INET6) → fails → existing subdomain fallback
2. getaddrinfo("localhost", AF_INET6) → fails on Android → new: Android IPv6 fallback
3. getaddrinfo("ip6-localhost", AF_INET6) → succeeds

This is applied to both the sync (GetHostEntryOrAddressesCore) and async (GetAddrInfoWithTelemetryAsync) code paths.

Testing

• macOS: 131 tests pass, 0 failures
• Android arm64 (emulator): all tests pass (XHarness exit code 0), including the 4 previously-disabled RespectsAddressFamily test cases
• The [ActiveIssue] annotations for DNS localhost subdomain tests with AddressFamily fail on Android #124751 are removed from both test methods

[dotnet-policy-service]

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[Copilot]

Pull request overview

Fixes Android-specific failures in RFC 6761 *.localhost subdomain resolution when an AddressFamily filter is applied (notably InterNetworkV6), by adding an Android IPv6 localhost fallback that retries using ip6-localhost if localhost IPv6 resolution fails.

Changes:

• Add Android-only IPv6 fallback: if resolving localhost with InterNetworkV6 fails with HostNotFound, retry with ip6-localhost (sync + async paths).
• Refactor async localhost-subdomain fallback to use helper local functions that include the Android retry behavior.
• Re-enable the Android variants of the RespectsAddressFamily tests by removing [ActiveIssue] annotations.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Net.NameResolution/src/System/Net/Dns.cs	Adds Android IPv6 retry (localhost → ip6-localhost) for InterNetworkV6 when localhost-subdomain fallback resolves plain localhost; applied to sync + async telemetry path.
src/libraries/System.Net.NameResolution/tests/FunctionalTests/GetHostEntryTest.cs	Removes Android [ActiveIssue] to re-enable localhost subdomain AddressFamily test coverage.
src/libraries/System.Net.NameResolution/tests/FunctionalTests/GetHostAddressesTest.cs	Removes Android [ActiveIssue] to re-enable localhost subdomain AddressFamily test coverage.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

[simonrozsival]

/azp run runtime-extra-platforms

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[wfurt]

I'm wondering if we should leave it as general fallback
My host file on Linux looks similar

cat /etc/hosts
127.0.0.1 localhost
# The following lines are desirable for IPv6 capable hosts
::1     ip6-localhost ip6-loopback

ping6  localhost
ping6: localhost: Address family for hostname not supported

Alternatively, if IPv6 resolution for localhost fails, we can fail the other lookups as well, e.g. update the test to deal with it. Any thoughts on this @dotnet/ncl ?