* Expand image url as well as link url, use robust Markdig

Rather than the brittle regex-based matching we were doing, switch to proper markdown processing so we can better support arbitrary content within the readme without potentially breaking due to the flaky nature of the regex.

We were also not expanding the image url if it contained a link, so now we're more consistent.

* Attempt to fix the broken test from obsolete MAUI Mac

* Split restore from build for better logging

* Avoid incompatible assembly from being copied to test output

This assembly seems to be a new transitive dep from some prior updates. By excluding it explicitly, we prevent hard-to-diagnose test failures in local builds