dotnet(nuget): add missing Sentry.Maui.CommunityToolkit.Mvvm

[jamescrosswell]

Context

• Same failure mode as dotnet(nuget): add missing Sentry.Hangfire #223 (Sentry.Hangfire) and Add Sentry.Extensions.AI nuget package #221 (Sentry.Extensions.AI)
• Follow-up to fix(serilog): logs from application namespaces beginning with "Sentry" are discarded sentry-dotnet#5456
• Tracking issue: 6.9.0 never reached the release registry: craft's registry target aborts on Sentry.Maui.CommunityToolkit.Mvvm sentry-dotnet#5495

Summary

Sentry.Maui.CommunityToolkit.Mvvm has shipped on NuGet since 5.8.0 but was never added here. getsentry/sentry-dotnet#5456 added it to sentry-dotnet's .craft.yml registry target, and the 6.9.0 publish then aborted (run 32010411662):

[error] "name" is required for new package "nuget:Sentry.Maui.CommunityToolkit.Mvvm".
        Add `name` to the registry target config in your .craft.yml.

Craft treats a package as new only when its latest.json is missing, so seeding the directory here makes the bare .craft.yml key work exactly as it does for the other 18 sentry-dotnet packages.

The failure isn't scoped to this one package — updateVersionInRegistry throws before craft commits anything, so the whole registry target rolls back. Every sentry-dotnet package is currently still pinned at 6.8.0 here, two releases into 6.9.0 being live on NuGet.

Notes for review

• Seeded at 6.8.0, not 6.9.0, deliberately. Craft aborts with Version file for "6.9.0" already exists. Aborting. if the version being published is already present — which would roll the target back again and leave all 19 packages stuck at 6.8.0. Seeding the previous version (which the package did ship) lets the retry create 6.9.0 and re-point the symlinks. dotnet(nuget): add missing Sentry.Hangfire #223 did the same, seeding 6.0.0-rc.2-prerelease.
• canonical has to match the .craft.yml key exactly or craft errors on the consistency check in getUpdatedManifest.
• created_at is the value craft wrote for the sibling packages in the 6.8.0 release, since it's the same release event.
• Symlinks generated with bin/sync-links, not by hand. I also re-ran the full make sync-all-links (via xargs, as the bare find blows the argument limit on macOS) and confirmed it reproduces the committed tree exactly, so the CI sync check should be clean.
• No sdks/sentry.dotnet.maui.communitytoolkit.mvvm symlink. Every other dotnet package has one, but this package doesn't emit its own sdk_info.name — its events go out as sentry.dotnet.maui — so the entry would name something nothing reports. Happy to add it if uniformity is preferred.

Relationship to getsentry/sentry-dotnet#5496

#5496 approaches the same failure from the other side: it gives sentry-dotnet's .craft.yml the name/packageUrl/mainDocsUrl that craft needs to create manifests itself — for every package, not just this one, so the next new package doesn't repeat this.

The two are complementary rather than exclusive. This PR unblocks the 6.9.0 retry now; #5496 stops it happening again. If both land, this package's .craft.yml entry is simply belt-and-braces — craft applies the config over the manifest on every release either way.