Add dotnet12 feeds to NuGet.config

[subhajitm]

Problem

Codeflow PR #11869 ([main] Source code updates from dotnet/dotnet) fails restore on every leg:

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

Same for Microsoft.NETCore.App.Runtime.win-{x86,x64,arm64} and Microsoft.NETCore.App.Host.win-{x86,x64,arm64}.

Cause

The VMR (dotnet/dotnet main) now publishes its 11.0.0-rc.1.* assets to the dotnet12 and dotnet12-transport feeds. Our NuGet.config only lists feeds up to dotnet11, so those packages are unreachable.

Verified against the live feeds:

Check	Result
11.0.0-rc.1.26452.110 on any of the 14 feeds we list	absent everywhere
11.0.0-rc.1.26411.119 (what main uses today)	present on dotnet11
Newest rc.1 available on dotnet11	11.0.0-rc.1.26431.118
11.0.0-rc.1.26452.110 on dotnet12	present (all 7 failing packages, plus crossgen2)
Microsoft.Private.Winforms 11.0.0-rc.1.26452.110	present on dotnet12-transport
dotnet/winforms NuGet.config	already lists both dotnet12 feeds

This is not a version-number problem and not a source bug — the referenced assets exist, just on a feed we do not consume yet.

Fix

Add the two dotnet12 sources, matching what dotnet/winforms already carries.

Validation

Applied on top of #11869 locally and ran Restore.cmd -ci for x86, x64 and arm64 — all three succeeded with 0 errors. All seven previously-NU1102 packages resolve at 11.0.0-rc.1.26452.110, and .nupkg.metadata confirms dotnet12 as the source.

Notes

• NuGet.config is meant to stay in sync with the dotnet-wpf-int repo; matching PRs are being raised there and in dnceng/internal/dotnet-wpf.
• Raised as a standalone change against main rather than editing the maestro-owned codeflow branch, since darc would overwrite hand-edits there. This unblocks every future main codeflow, not just [main] Source code updates from dotnet/dotnet #11869.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

🟢 Approval recommended

The change is a minimal, well-scoped NuGet feed addition that aligns with the stated restore failure cause and does not alter build logic.

Pull request overview

This PR updates the repository’s NuGet.config package sources to include the dotnet12 and dotnet12-transport Azure DevOps feeds so restores can resolve newly published VMR (dotnet/dotnet main) assets that are no longer present on dotnet11 feeds.

Changes:

• Add dotnet12 package source.
• Add dotnet12-transport package source.

File summaries

File	Description
NuGet.config	Adds dotnet12 and dotnet12-transport feeds to unblock restores that require packages published to those sources.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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