Validate Dns hostname for embedded null characters

[Copilot]

Dns.GetHostEntry and friends forwarded hostnames containing \0 to native resolvers (getaddrinfo/gethostbyname), which treat \0 as end-of-string. The name was silently truncated and resolution failed in confusing ways instead of being rejected up front.

Changes

• Dns.ValidateHostName — added a Contains('\0') check that throws ArgumentException with parameter name hostName. This is the single funnel reached from both the sync (GetHostEntryOrAddressesCore) and async (GetHostEntryOrAddressesCoreAsync) string code paths, so one check covers GetHostEntry, GetHostAddresses, their Async/Begin variants, and the obsolete GetHostByName/Resolve overloads. IP-literal fast paths are unaffected (an IP literal cannot contain \0).
• Strings.resx — added net_hostname_invalid_character ("The host name cannot contain a null character.").
• GetHostEntryTest.cs — added DnsGetHostEntry_NullCharacterInName_ThrowsArgumentException theory exercising leading/trailing/embedded null patterns against the sync and async entry points.

// Before: silently truncated, then SocketException from the OS resolver
Dns.GetHostEntry("foo\0bar");

// After:
// System.ArgumentException: The host name cannot contain a null character. (Parameter 'hostName')

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