Foundation-PR: pick the nightly by artifact presence, not build result

[romsi1983]

Port of the release/dev/monobuild fix. The two channels fail in opposite directions but for the same reason: runVersion: 'latestFromBranch' selects the nightly by WHOLE-BUILD result, which is not the property the download depends on. The monobuild nightly is routinely failed purely because of its Test Gate stage while its Aggregate stage still publishes a correctly named artifact.

• release/dev/monobuild has allowFailedBuilds: false, so it rejects every Test-Gate-red nightly and falls back to a run old enough that retention has purged its artifacts. That is the reported break: "Artifact WindowsAppSDK-NuGet-And-MSIX was not found for build 150121328."
• this branch has allowFailedBuilds: true, so it takes the newest run unconditionally - including runs whose Aggregate stage never produced the artifact. 7 of the 15 most recent nightlies on this branch publish no WindowsAppSDK-NuGet-And-MSIX at all, so the download is a coin flip today and fails with the same 404 whenever it lands on one of them.

Fix: resolve the build id from artifact presence instead. A new shared steps template probes the Build REST API for the newest completed run on the branch that publishes the exact artifact name, then downloads with runVersion: 'specific' / runId. It carries a recency guard (maxNightlyAgeInDays, default 7) that fails with "No run of pipeline ... in the last N days publishes an artifact named ..." rather than a confusing 404, and a NightlyBuildIdOverride queue-time variable to pin a known-good nightly.

The added lines are byte-identical to the release/dev/monobuild change and the new template file is the same blob; the only difference between the two commits is which allowFailedBuilds value and comment block was removed.

Note for reviewers, deliberately NOT changed here: NightlyBranchName still defaults to 'refs/heads/release/dev/monobuild', i.e. this branch's Foundation builds consume the MAIN monobuild channel's package even though the nightly does run on release/dev/monobuild-2.0-stable and does publish the artifact there. That looks wrong but it is a separate behavioural decision for the 2.0-stable owners, not part of this fix.

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