[DO NOT MERGE] CI validation for #11893 (dotnet12 feeds vs codeflow #11869)

[subhajitm]

Do not merge. CI-only validation branch for #11893, to be closed once green.

Why this exists

#11893 adds the dotnet12 / dotnet12-transport feeds to NuGet.config. Its own CI run cannot actually prove the fix, because main currently pins 11.0.0-rc.1.26411.119, which is still available on dotnet11 — so #11893 going green only demonstrates that adding the feeds causes no regression.

This branch is codeflow PR #11869 + the NuGet.config change from #11893, so CI here restores the exact versions that are failing today (11.0.0-rc.1.26452.110). A green run is direct evidence that the feed addition fixes the NU1102 failures on #11869.

What is being reproduced

#11869 fails on all six legs with:

Tools.proj : error NU1102: Unable to find package Microsoft.NETCore.App.Ref
             with version (= 11.0.0-rc.1.26452.110)

plus Microsoft.NETCore.App.Runtime.win-{x86,x64,arm64} and Microsoft.NETCore.App.Host.win-{x86,x64,arm64}. Those assets are published to dotnet12 / dotnet12-transport, which WPF does not currently consume.

Locally, Restore.cmd -ci with this branch succeeds for x86, x64 and arm64 with 0 errors, and all seven packages resolve from dotnet12. This PR confirms the same in real CI.

See #11893 for the full analysis and the change actually proposed for merge.

Microsoft Reviewers: Open in CodeFlow

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