Use dnssd for DnsResolver on OSX - #134141

Move DnsSdRecord and the DNSServiceQueryRecord rdata parsers out of
DnsResolverPal.OSX into a new DnsSdRecordParsing static class. Link the
parsing file (and DnsRecords.cs) into System.Net.NameResolution.Unit.Tests
alongside the other production parsers, and add direct unit tests for the
interface-index handling, root name / MX / SRV parsing, TXT framing, and
name-validation edge cases.

The reflection-based tests in DnsResolverTest.cs that reached into private
PAL members are removed in favor of the new unit tests.

…ask.Run

The previous async path wrapped the blocking Poll loop in Task.Run, which
pinned a thread-pool thread for the whole query duration. Wrap the
mDNSResponder fd (returned by DNSServiceRefSockFD) in a non-owning
System.Net.Sockets.Socket via the existing DnsSocket reflection cache and
await Socket.ReceiveAsync(Memory<byte>.Empty, ct) as a real async POLLIN.
DNSServiceProcessResult is still called synchronously when the wait
completes to consume + dispatch the record via the callback.

The sync path keeps Interop.Sys.Poll — a sync caller has already committed
a thread to blocking, and adding async plumbing there would only add
moving parts. Pre-canceled tokens on the async path return
Task.FromCanceled to preserve the TaskCanceledException surface the old
Task.Run(action, ct) shortcut produced.

Addresses feedback from teo-tsirpanis in dotnet#131934.

- Move the embedded-NUL check into DnsResolver.ValidateName so every
  platform rejects NUL-injected names (Windows DnsQueryEx and macOS
  DNSServiceQueryRecord both take null-terminated strings), and drop the
  OSX-only copy.
- Widen DNS labels byte-by-byte in DnsSdRecordParsing.TryParseDnsName
  instead of Encoding.UTF8.GetString, matching how the managed resolver
  decodes response labels (deterministic output for non-UTF-8 bytes).
- Loosen ResolveAddresses_NonExistent_ReturnsNxDomain and the OSX
  CompletesPromptly variant to accept either NoError or NxDomain, since
  mDNSResponder can report NXDOMAIN as NoSuchName or NoSuchRecord
  depending on version.

- Interop.Dnssd now marshals the DNSServiceRef as SafeDnsServiceHandle
  instead of raw IntPtr for DNSServiceQueryRecord/RefSockFD/ProcessResult,
  so handle lifetime is managed by the LibraryImport source generator.
  Move SafeDnsServiceHandle to file scope (internal) with a parameterless
  ctor so the source generator can construct it for out params.
- Collapse the OSX PAL's Query/QueryCore/QueryRecord sync/async pairs
  into single methods that take bool async and branch only at the actual
  wait (Interop.Sys.Poll vs DnsSocket.WaitReadableAsync). Pre-canceled
  check moves up into Query.
- DnsSocket.WaitReadableAsync: switch from Socket.ReceiveAsync with an
  empty buffer to a 1-byte SocketFlags.Peek. An empty-buffer receive
  completes synchronously with zero bytes on Unix (0-byte recv returns
  immediately without ever waiting for POLLIN), so we'd have busy-looped
  calling DNSServiceProcessResult without data. Peek leaves the byte in
  the socket for DNSServiceProcessResult to consume. Dispose the scratch
  Socket via ((IDisposable)socket).Dispose() instead of a reflected
  Dispose delegate.

Verified locally with the OuterLoop async DNS-SD tests (A/AAAA/CNAME
chain, IPv4-only, non-existent, SRV) on macOS arm64 - all 12 pass.

A CNAME query can legitimately return NODATA (NoError with no records)
if the name only has A/AAAA records, so require only that any returned
records are well-formed.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>