[Ignore, gh stacks tests] Implement DnsResolver for MacOS

[rzikm]

Raising PR in my fork to try out githubs stacked PRs feature

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[Copilot]

Pull request overview

This PR adds a macOS implementation of DnsResolver (via DNSServiceQueryRecord) and introduces a managed stub resolver + DNS wire-format parsing/writing helpers to support Unix/macOS scenarios and loopback testing. It also expands the test matrix to include -osx TFMs and adds substantial unit/functional coverage for DNS message handling and record parsing.

Changes:

• Add DnsResolverPal.OSX using Interop.Dnssd (DNSServiceQueryRecord) for system-configured DNS resolution on macOS.
• Add a managed stub resolver (DnsResolverPal.Managed) plus DNS wire-format types (encoded name, header/reader/writer, record parsers) and /etc/resolv.conf parsing.
• Add new unit tests for DNS wire encoding/parsing, update PAL/functional tests, and add -osx TFMs to relevant projects.

Reviewed changes

Copilot reviewed 24 out of 24 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/libraries/System.Net.NameResolution/tests/UnitTests/System.Net.NameResolution.Unit.Tests.csproj	New unit test project linking wire-format production code for focused unit coverage.
src/libraries/System.Net.NameResolution/tests/UnitTests/DnsRecordTypeTests.cs	Adds unit tests for typed DNS RDATA parsing (A/AAAA/CNAME/MX/SRV/TXT/PTR/NS/SOA) including malformed cases.
src/libraries/System.Net.NameResolution/tests/UnitTests/DnsMessageWriterTests.cs	Adds unit tests validating DNS query writer output and buffer-boundary behavior.
src/libraries/System.Net.NameResolution/tests/UnitTests/DnsMessageReaderTests.cs	Adds unit tests for parsing DNS responses, compression pointers, and truncation/malformed handling.
src/libraries/System.Net.NameResolution/tests/UnitTests/DnsMessageHeaderTests.cs	Adds unit tests for DNS header encoding/decoding and round-trips.
src/libraries/System.Net.NameResolution/tests/UnitTests/DnsEncodedNameTests.cs	Adds unit tests for DNS name encoding/decoding, pointer rules, and IDN behavior.
src/libraries/System.Net.NameResolution/tests/PalTests/System.Net.NameResolution.Pal.Tests.csproj	Includes ResolvConf and its tests for Unix PAL coverage.
src/libraries/System.Net.NameResolution/tests/PalTests/ResolvConfTests.cs	Adds tests for /etc/resolv.conf “nameserver” parsing behavior.
src/libraries/System.Net.NameResolution/tests/FunctionalTests/System.Net.NameResolution.Functional.Tests.csproj	Adds -osx TFM to functional test target frameworks.
src/libraries/System.Net.NameResolution/tests/FunctionalTests/DnsResolverTest.cs	Expands platform coverage to Windows+macOS and adds macOS-specific behavior assertions.
src/libraries/System.Net.NameResolution/tests/FunctionalTests/DnsResolverLoopbackTest.cs	Broadens loopback coverage beyond Windows and adds macOS managed-resolver verification for custom servers.
src/libraries/System.Net.NameResolution/src/System/Net/ResolvConf.cs	Adds /etc/resolv.conf parsing + caching for system DNS servers on Unix-like platforms.
src/libraries/System.Net.NameResolution/src/System/Net/DnsWireEnums.cs	Adds internal DNS wire enums (TYPE/CLASS/OPCODE/flags).
src/libraries/System.Net.NameResolution/src/System/Net/DnsSocket.cs	Adds reflection-based socket wrapper to avoid a static dependency cycle on System.Net.Sockets.
src/libraries/System.Net.NameResolution/src/System/Net/DnsResolverPal.OSX.cs	Adds macOS PAL using DNS-SD APIs for system-policy DNS resolution when no custom servers are configured.
src/libraries/System.Net.NameResolution/src/System/Net/DnsResolverPal.Managed.cs	Adds managed stub resolver for Unix/macOS with UDP+TCP fallback, response validation, and record parsing.
src/libraries/System.Net.NameResolution/src/System/Net/DnsRecordParsing.cs	Adds typed record parsing helpers over DnsRecord RDATA.
src/libraries/System.Net.NameResolution/src/System/Net/DnsMessageWriter.cs	Adds DNS query writer for header + question section.
src/libraries/System.Net.NameResolution/src/System/Net/DnsMessageReader.cs	Adds DNS message reader for header/questions/records with compression-aware name parsing.
src/libraries/System.Net.NameResolution/src/System/Net/DnsMessageHeader.cs	Adds encoding/decoding for the fixed 12-byte DNS header.
src/libraries/System.Net.NameResolution/src/System/Net/DnsEncodedName.cs	Adds DNS wire-format name type with compression-pointer support and IDN handling.
src/libraries/System.Net.NameResolution/src/System.Net.NameResolution.csproj	Adds -osx TFM and wires managed + macOS PAL sources/interop into the build.
src/libraries/System.Net.NameResolution/src/Resources/Strings.resx	Adds new resource strings used by the managed resolver.
src/libraries/Common/src/Interop/OSX/Interop.Dnssd.cs	Adds DNS-SD interop declarations and constants for macOS resolver implementation.

Suppressed comments (1)

src/libraries/System.Net.NameResolution/src/System/Net/DnsResolverPal.Managed.cs:809

• Throwing a parameterless IOException here makes failures much harder to diagnose. Prefer an IOException with a resource string (similar to the ReceiveExact* path) so callers get an actionable error message.

                int sent = socket.Send(buffer.Slice(totalSent));
                if (sent == 0)
                {
                    throw new IOException();
                }

[rzikm]

PullRequest.base is invalid
✗ Cannot create stack: Pull requests from forks cannot be added to stacks.

looks like stacks are not supported across forks yet