Fix PR Details diff rendering and reduce initial load stalls

[DomDom3333]

Summary

• fixed diff content loading cancellation so opening one file no longer cancels another file’s in-flight load
• fixed DiffContainer discovery in PullRequestDetailsWindow by targeting the named container instead of the first descendant Border
• removed unused per-load cancellation state from PullRequestDetailsWindowViewModel

Root cause

1. Each expand action called BeginDiffContentLoad(), which canceled the previous token globally. During expand-all or fast clicks this canceled active loads, so many diff viewers never received content.
2. CreateDiffViewerAsync used expander.FindDescendantOfType<Border>(), which can return a header border instead of the actual content host, so the DiffViewer was sometimes injected into the wrong place.

Validation

• dotnet build AzurePrOps/AzurePrOps.sln -c Release
• dotnet build AzurePrOps/AzurePrOps.sln -c Release --no-restore
• dotnet test AzurePrOps/AzurePrOps.Tests/AzurePrOps.Tests.csproj -c Release --no-build

Build/test passed; only pre-existing warnings remain.

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Codex Task