[repo] Test/CI coverage for mobile/MAUI/Xamarin

[CodeBlanch]

Relates to #5775

We have some code to avoid PlatformNotSupportedExceptions being thrown running on "mobile" platforms: #5821.

The issue is there aren't any unit tests or CI legs/workflows testing/validating this code.

High-level what we want is coverage for these "mobile" platforms, maybe others. Ideally a CI tweak. I say "ideally" because I spent some time on this and it doesn't seem simple to run unit tests for mobile.

What we don't want is to force every dev who wants to work on OTel .NET to become a mobile developer. The core dev experience shouldn't involve installing mobile workloads/sdks/simulators/emulators etc. We could have a dedicated solution to contain MAUI apps though. Open to ideas.

Looking for help on this from anyone with good experience in running .NET on mobile 😄