Re-enable localhost loopback assertions on Apple mobile and Android

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The *Localhost* NameResolution tests were guarded by requireLoopback = !PlatformDetection.IsAppleMobile && !PlatformDetection.IsAndroid, which skipped the loopback assertions on those platforms. The underlying resolver issue was addressed by #128068, so these gates can be removed to surface any remaining regressions.

• GetHostAddressesTest.cs: drop the requireLoopback gate in DnsGetHostAddresses_LocalhostSubdomain_ReturnsLoopback, DnsGetHostAddresses_LocalhostAndSubdomain_BothReturnLoopback, and DnsGetHostAddresses_LocalhostSubdomainWithTrailingDot_ReturnsLoopback; assert loopback unconditionally.
• GetHostEntryTest.cs: same change for the three corresponding DnsGetHostEntry_* tests.
• Outdated comments referencing the Apple mobile / Android skip are removed.

// before
bool requireLoopback = !PlatformDetection.IsAppleMobile && !PlatformDetection.IsAndroid;
IPAddress[] addresses = Dns.GetHostAddresses(hostName);
Assert.True(addresses.Length >= 1, "Expected at least one address");
if (requireLoopback)
{
    Assert.All(addresses, addr => Assert.True(IPAddress.IsLoopback(addr), $"Expected loopback address but got: {addr}"));
}

// after
IPAddress[] addresses = Dns.GetHostAddresses(hostName);
Assert.True(addresses.Length >= 1, "Expected at least one address");
Assert.All(addresses, addr => Assert.True(IPAddress.IsLoopback(addr), $"Expected loopback address but got: {addr}"));

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