[mdns] allow resolving IPv4 address for host or service with mDNSResponder

[librasungirl]

In mdns_mdnssd.cpp, DNSServiceGetAddrInfo is called with kDNSServiceProtocol_IPv6 | kDNSServiceProtocol_IPv4 for both service instance resolution and host subscription resolution. This indicates that either an IPv6 or an IPv4 address is explicitly expected and allowed from the mDNS responder.

However, the callback handlers HandleGetAddrInfoResult and HandleResolveResult previously discarded all non-IPv6 addresses with VerifyOrExit(aAddress->sa_family == AF_INET6). This was a bug that caused hosts and services advertising only IPv4 A records to be ignored rather than resolved.

This change fixes the bug by:

• Accepting both AF_INET and AF_INET6 address families in PublisherMDnsSd::ServiceInstanceResolution::HandleGetAddrInfoResult and PublisherMDnsSd::HostSubscription::HandleResolveResult.
• Extending Ip6Address with CopyFrom(const struct sockaddr &), CopyFrom(const struct sockaddr_in &) and CopyFrom(const struct in_addr &) to convert resolved IPv4 addresses into IPv4-mapped IPv6 addresses (::ffff:a.b.c.d), consistent with OpenThread's DNS-SD / discovery proxy specification.
• Adding unit test for Ip6Address covering IPv4-mapped addresses.

[gemini-code-assist]

Code Review

This pull request introduces support for IPv4-mapped IPv6 addresses in the Ip6Address class, adding helper methods to copy addresses from sockaddr, sockaddr_in, and in_addr structures, along with corresponding unit tests. It also updates the mDNS publisher to handle these address types. The review feedback highlights a potential null pointer dereference of aAddress in src/mdns/mdns_mdnssd.cpp and suggests avoiding union type punning in src/common/types.cpp to prevent undefined behavior and endianness issues.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.97%. Comparing base (2b41187) to head (0b0c5ee).
⚠️ Report is 1644 commits behind head on main.

Files with missing lines	Patch %	Lines
tests/gtest/test_common_types.cpp	36.17%	30 Missing ⚠️
src/mdns/mdns_mdnssd.cpp	42.10%	9 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3557      +/-   ##
==========================================
- Coverage   55.77%   47.97%   -7.80%     
==========================================
  Files          87      149      +62     
  Lines        6890    18514   +11624     
  Branches        0     1496    +1496     
==========================================
+ Hits         3843     8883    +5040     
- Misses       3047     9015    +5968     
- Partials        0      616     +616     

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