[CI] NU1102: Microsoft.NETCore.App 9.0.21 packs unavailable in dependency-update PRs

## Failure signature

Dependency-update PRs fail while restoring `Xamarin.Messaging.Build.csproj` / `Xamarin.MacDev.Tasks.slnx`:

```text
error NU1102: Unable to find package Microsoft.NETCore.App.Ref with version (= 9.0.21)
error NU1102: Unable to find package Microsoft.NETCore.App.Host. with version (= 9.0.21)
```

The affected feeds report `9.0.20` as the nearest runtime-pack version. The host-pack RID is `osx-arm64` on macOS and `linux-x64` on Linux.

## Confirmed occurrences

| PR / head | Azure run, attempt, job, and task | Platform |
|---|---|---|
| #26443 / `54b73e1a5c29574656ccfd8ae63b05aa293e334c` | [15099802 attempt 1 - Build packages / Build](https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=15099802&view=logs&j=600d49c6-e96b-5e8b-2c3a-9f351e2e5848&t=80a136b1-24dd-5d7f-8552-895fbffe54ed) | macOS |
| #26443 / same head | [15099804 attempt 1 - API diff / Build](https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=15099804&view=logs&j=276491e5-a85e-51e3-6c38-98e261218d1a&t=76afb9de-7feb-5cd3-1173-f84cb7d12f5e) | macOS |
| #26443 / same head | [15099800 attempt 1 - Build packages / Build](https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=15099800&view=logs&j=600d49c6-e96b-5e8b-2c3a-9f351e2e5848&t=80a136b1-24dd-5d7f-8552-895fbffe54ed) | macOS |
| #26443 / same head | [15099802 attempt 1 - Linux Build Verification / Build on Linux](https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=15099802&view=logs&j=cd95c9ab-ceee-5f36-46bb-743e43fba63c&t=e8599f2d-16cf-5ca3-8351-5ddd36575f8c) | Linux |
| #26443 / `b2c2bcc5bc4ec5f63c07a6fe8530fd3b8d6a4723` | [15114159 attempt 1 - Build packages / Build](https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=15114159&view=logs&j=600d49c6-e96b-5e8b-2c3a-9f351e2e5848&t=80a136b1-24dd-5d7f-8552-895fbffe54ed) | macOS |
| #26443 / same head | [15114164 attempt 1 - Build packages / Build](https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=15114164&view=logs&j=600d49c6-e96b-5e8b-2c3a-9f351e2e5848&t=80a136b1-24dd-5d7f-8552-895fbffe54ed) | macOS |
| #26443 / same head | [15114170 attempt 1 - API diff / Build](https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=15114170&view=logs&j=276491e5-a85e-51e3-6c38-98e261218d1a&t=76afb9de-7feb-5cd3-1173-f84cb7d12f5e) | macOS |
| #26443 / same head | [15114164 attempt 1 - Linux Build Verification / Build on Linux](https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=15114164&view=logs&j=cd95c9ab-ceee-5f36-46bb-743e43fba63c&t=e8599f2d-16cf-5ca3-8351-5ddd36575f8c) | Linux |
| #26444 / `6a7fb6a417e516bb97b6f91d813645b5eadff309` | [15122717 attempt 1 - Build packages / Build](https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=15122717&view=logs&j=600d49c6-e96b-5e8b-2c3a-9f351e2e5848&t=80a136b1-24dd-5d7f-8552-895fbffe54ed) | macOS |
| #26444 / same head | [15122722 attempt 1 - API diff / Build](https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=15122722&view=logs&j=276491e5-a85e-51e3-6c38-98e261218d1a&t=76afb9de-7feb-5cd3-1173-f84cb7d12f5e) | macOS |
| #26444 / same head | [15122717 attempt 1 - Linux Build Verification / Build on Linux](https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=15122717&view=logs&j=cd95c9ab-ceee-5f36-46bb-743e43fba63c&t=e8599f2d-16cf-5ca3-8351-5ddd36575f8c) | Linux |
| #26444 / same head | [15122707 attempt 1 - Build packages / Build](https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=15122707&view=logs&j=600d49c6-e96b-5e8b-2c3a-9f351e2e5848&t=80a136b1-24dd-5d7f-8552-895fbffe54ed) | macOS |

**Total:** 12 confirmed root-task occurrences across 2 distinct monitored PRs.

First confirmed occurrence: `2026-08-26T02:26:08Z`. Latest confirmed occurrence: `2026-08-27T20:18:59Z`.

## Classification and impact

**Source/configuration-related deterministic restore failure.** The same package/version mismatch reproduces across macOS and Linux workers, three pipelines, two PRs, and two successive #26443 heads. Build and test execution cannot proceed.

The API-diff jobs fail during the prerequisite build. Their later missing `change-detection.zip` errors are downstream cascades, not current-PR or stable-baseline API compatibility results. No API binding or suppression-file change is indicated by this failure.

## Investigation performed

- Inspected Azure build metadata, full timelines, root task logs, and aggregate failures for every occurrence.
- Verified the exact reference-pack and platform host-pack `NU1102` errors, requested version `9.0.21`, and nearest runtime-feed version `9.0.20`.
- Searched all open and closed `dotnet/macios` issues separately by exact signature, package names, error code and fragments, task/project, component, and version.
- The closest predecessor is #26465, which tracks the same mechanism for the distinct exact version `9.0.20`; its affected heads were fixed before this `9.0.21` signature appeared.
- No retries were submitted: unchanged heads deterministically request the same unavailable package versions.
- Issue-specific checklist: not applicable; no existing exact-signature issue had an **Additional info to collect** section.

## Workaround

Align the dependency and package-source configuration, or make the exact `9.0.21` reference and platform host packs available in the runtime feed used by these dependency-update PRs. Do not retry unchanged heads merely to see whether restore passes.

Azure logs and artifacts are retention-limited.

Start by reproducing restore for Xamarin.Messaging.Build.csproj and Xamarin.MacDev.Tasks.slnx in the affected Azure Build packages and Linux Build Verification jobs. Inspect the dependency and package-source configuration used by dependency-update PRs, then verify that Microsoft.NETCore.App 9.0.21 reference and host packs resolve on macOS and Linux and the prerequisite builds complete without downstream API-diff errors.

Written by the indexing model from the issue text.