The Managed DNS implementation correctly special-cased "localhost" and resolved to loopback without contacting DNS server, but the authoritative RFC 6761 requires the same for subdomains as well (e.g. service.localhost). Ths PR fills the gap, including the other special-cased name "invalid".

Implement RFC6761 reserved DNS names handling

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October 15, 2025 11:35

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Pull Request Overview

This PR implements RFC 6761 compliant handling of reserved DNS names by extending special-case treatment beyond just "localhost" to include its subdomains (e.g., "service.localhost") and the "invalid" domain. The implementation centralizes reserved name checking and ensures DNS servers are not contacted for these special cases.

Key Changes:

Refactored localhost resolution logic into a sharedCheckIsReservedDnsName method that handles both "localhost" and "invalid" domains plus their subdomains
Added comprehensive test coverage for reserved name resolution including subdomain variants and explicit root labels

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Libraries/Microsoft.Extensions.ServiceDiscovery.Dns/Resolver/DnsResolver.cs	Refactored hardcoded localhost checks into centralized RFC 6761 reserved name handling with support for subdomains and the "invalid" domain
test/Libraries/Microsoft.Extensions.ServiceDiscovery.Dns.Tests/Resolver/ResolveAddressesTests.cs	Added test cases for subdomain variants of reserved names and renamed test to reflect broader scope