Add async UnpackStreamAsync to ISyncManagementService, obsolete sync UnpackStream

[KevinJump]

What

ISyncManagementService gets a new UnpackStreamAsync(Stream stream) method. The existing UnpackStream(Stream stream) is now [Obsolete] with a default implementation that forwards to the async version.

Why

uSyncManagementService.UnpackStream called the obsolete ISyncActionService.UnpackImportFromStream(Stream) (CS0618, scheduled for removal in v19), producing the last remaining build warning. ISyncActionService already exposes the async replacement (UnpackImportFromStreamAsync), so the fix is to plumb that all the way up through ISyncManagementService and its controller caller, rather than keep calling the obsolete sync member.

How

Mirrors the pattern ISyncActionService already uses for this exact same rename (UnpackImportFromStream → UnpackImportFromStreamAsync, see uSync.BackOffice/Services/ISyncActionService.cs:71-78) and the existing [Obsolete("... will be removed in vXX")] convention already used elsewhere in ISyncManagementService (GetActions()):

• ISyncManagementService.cs — added Task<UploadImportResult> UnpackStreamAsync(Stream stream); UnpackStream is now [Obsolete] with a default body calling UnpackStreamAsync(stream).Result.
• uSyncManagementService.cs — UnpackStream implementation replaced with UnpackStreamAsync, which now awaits _syncActionService.UnpackImportFromStreamAsync(stream) directly instead of the obsolete sync wrapper.
• uSyncPerformActionController.cs — ProcessUpload now calls await _managementService.UnpackStreamAsync(stream).

No other call sites reference UnpackStream.

Notes for reviewers

• Full solution builds with 0 errors and 0 CS0618 warnings (down from the last remaining one). Only a pre-existing, unrelated CS8632 nullable-annotation warning remains in the test project.
• All 148 existing tests pass.
• Minor style note: ISyncActionService's equivalent default body uses .GetAwaiter().GetResult() rather than .Result (avoids wrapping exceptions in AggregateException) — worth aligning if this ever gets called from a sync context, but since UnpackStream has no callers left in-repo it's not a functional concern here.

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