Stable failure notifications

Currently, every failure creates a new GitHub issue. If the same failure occurs repeatedly (e.g. AutoBuilder fails multiple times for the same manifest/repo), duplicate issues accumulate.

Instead, `NotificationService` should detect an existing open issue for the same failure and update it (e.g. add a comment) rather than opening a new one. The existing notification metadata mechanism could be extended to support this:

https://github.com/dotnet/docker-tools/blob/cb52d339baaa7b2dafffa5b8141b4ebde2e509a4/src/ImageBuilder/NotificationHelper.cs#L11-L14

A deduplication key (e.g. subscription name or manifest/repo) could be added to the metadata. Before creating a new issue, `NotificationService` would search for an open issue with a matching key and append a comment instead of filing a duplicate.