Repin RadiusDeployTests KIND container image off the moving `dotnet/samples:aspnetapp` tag

## Background

The KIND-based CLI E2E test `RadiusDeployTests.DeployRadiusContainerToKind` (added in #18730) uses a moving, deprecated sample image:

```csharp
// tests/Aspire.Cli.EndToEnd.Tests/RadiusDeployTests.cs
private const string ContainerImage = "mcr.microsoft.com/dotnet/samples:aspnetapp";
private const int ContainerPort = 8080;
```

Per [dotnet/dotnet-docker#7191](https://github.com/dotnet/dotnet-docker/issues/7191) and the `README.samples.md` support notes, the `dotnet/samples` images are **not stable and can break at any time**. This repo already migrated away from them elsewhere — see `tests/Aspire.Deployment.EndToEnd.Tests/AcaCompactNamingDeploymentTests.cs`, which now uses:

```csharp
builder.AddContainer("worker", "mcr.microsoft.com/azuredocs/aci-helloworld", "latest")
.WithImageSHA256("456a1150aa41340a14c7be1342deda2cde9e6e7df9fde6b8a69de0ae04f92fad")
```

This was raised in review on #18730 and **deferred** to avoid regressing a currently-green test as part of an otherwise low-risk review-fix commit.

## Proposed change

Migrate `RadiusDeployTests` to the same convention:

- `ContainerImage` → `mcr.microsoft.com/azuredocs/aci-helloworld`
- `ContainerPort` `8080` → `80` (flows through `WithHttpEndpoint(targetPort:)` and the `18080:{ContainerPort}` port-forward; curl on `/` still returns 200)
- Add `.WithImageSHA256("456a1150aa41340a14c7be1342deda2cde9e6e7df9fde6b8a69de0ae04f92fad")` to the generated AppHost wiring

## Things to verify

- Whether `WithImageSHA256(...)` flows the digest into the Radius-generated `app.bicep` (if not, the tag pin to a stable image still mitigates the primary instability concern, but the digest pin would be a no-op for the deployed pod).
- That `Assert.Contains(ContainerImage, appBicep)` still holds with the new image reference.
- This KIND E2E can only be re-validated in CI (amd64 runner); `aci-helloworld`'s image index is amd64-only, which is acceptable for CI but worth noting.

## Context

- PR: #18730
- Reference convention: `tests/Aspire.Deployment.EndToEnd.Tests/AcaCompactNamingDeploymentTests.cs`
- File to change: `tests/Aspire.Cli.EndToEnd.Tests/RadiusDeployTests.cs`

Start in tests/Aspire.Cli.EndToEnd.Tests/RadiusDeployTests.cs, focusing on RadiusDeployTests.DeployRadiusContainerToKind and its ContainerImage and ContainerPort constants. Compare the generated AppHost wiring with tests/Aspire.Deployment.EndToEnd.Tests/AcaCompactNamingDeploymentTests.cs, then verify the image reference and digest in the generated app.bicep. Re-validate the KIND E2E test in CI on an amd64 runner.

Written by the indexing model from the issue text.