Fix SamplesCompatTest NU1605: insert missing PackageVersion entries when rewriting samples' Directory.Packages.props

[v-arlysenko]

Summary

Fixes an NU1605 build break in BuildSampleApps_x64 / BuildSampleApps_arm64
of TransportPackage-Foundation-Nightly (e.g. build 146281092):

NU1605: Detected package downgrade: Microsoft.WindowsAppSDK.Base
        from 2.0.260223001-experimental to 2.0.3-dev.experimental1
Unpackaged -> Microsoft.WindowsAppSDK.Foundation -> Microsoft.WindowsAppSDK.Base (>= 2.0.260223001-experimental)
Unpackaged -> Microsoft.WindowsAppSDK.Base (>= 2.0.3-dev.experimental1)

Internal work item: AB#62180574

Root cause

test/ModifySampleAppsReferences.ps1 rewrites WASDK package versions in WindowsAppSDK-Samples@release/experimental at pipeline time, using the Foundation transport package's extracted contents as the source of truth.

For Directory.Packages.props it only replaced existing PackageVersion Include="…" Version="…" entries — it never inserted missing ones. The samples branch doesn't declare a PackageVersion for Microsoft.WindowsAppSDK.Base, so the script silently did nothing for that package. NuGet restore then resolved Base from the transport-package metadata, picking an older 2.0.3-dev.experimental1 that conflicts with Foundation's pinned >= 2.0.260223001-experimental → NU1605.

Fix

When rewriting Directory.Packages.props, if the PackageVersion Include="X" entry is missing, insert <PackageVersion Include="X" Version="…" /> into the first <ItemGroup>. Existing entries are still updated in place.

packages.config handling is unchanged; the samples failing in this bug all use central package management. We can add symmetric insert logic later if a packages.config sample hits the same NU1605.

Validation

Local dry-run of the script against a clone of release/experimental using the same $packagesToUpdateTable the pipeline builds: the rewritten Samples/Directory.Packages.props now contains a <PackageVersion Include="Microsoft.WindowsAppSDK.Base" Version="…" /> entry matching the Foundation transport package version.

Risk

Low — change is confined to a build-time script that runs only inside TransportPackage-Foundation-Nightly against a cloned samples repo; no shipping code or sample source is modified.

[Scottj1s]

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[v-arlysenko]

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