Extract FanArtTV telemetry contracts

[cjaliaga]

Summary

• add the separately packable TrackSeries.FanArtTV.Client.Diagnostics contract package
• make the client and OpenTelemetry integration consume the shared telemetry names without an OpenTelemetry-to-client dependency
• preserve the existing public telemetry type through a main-client type forwarder
• build, pack, and publish all three packages together

Validation

• Release build: 0 warnings, 0 errors for net8.0 and net10.0
• Tests: 28 passed across both target frameworks (26 unit + 2 integration)
• Stable packages produced: all three at 2.0.4
• Diagnostics nuspec: no package dependencies
• OpenTelemetry nuspec: TrackSeries.FanArtTV.Client.Diagnostics 2.0.4 and OpenTelemetry.Extensions.Hosting 1.17.0; no main client dependency
• Consumer restore graph: Diagnostics and OpenTelemetry only; TrackSeries.FanArtTV.Client absent
• PR Build: passed

Release

This targets the v2.0 maintenance branch so NBGV assigns the next stable version, 2.0.4.

Use squash merge or rebase merge. Do not create a merge commit: an explicit NBGV simulation showed that a merge commit would add a second version-height commit and produce 2.0.5; squash/rebase keeps the release at 2.0.4.

After review and squash/rebase merge, verify dotnet nbgv get-version -v NuGetPackageVersion returns 2.0.4, then create and push the annotated v2.0.4 tag on that commit to trigger the established NuGet publishing workflow.