DNX Acquisition Mode fails when offline

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

If you configure `AspireCliInvocationMode` to use DNX, the `dotnet run` will fail if you have no network connectivity. This is probably root caused by dotnet/sdk#50579 .

### Expected Behavior

Aspire should be able fall back to CLI versions previosuly acquired by DNX

### Steps To Reproduce

1. Enable dnx invocation
```

true
DnxPinned

```
2. Turn on flight mode / turn off wifi / unplug all ethernet connections.

### Exceptions (if any)

dotnet run
```
> dotnet run --project .\app\REDACTED.AppHost\
Using launch settings from .\app\REDACTED.AppHost\Properties\launchSettings.json...
Building...
failed with 1 error(s) (6.8s)
S:\_packages\nuget\aspire.hosting.apphost\13.5.3\build\Aspire.Hosting.AppHost.targets(558,5): error DNX could not restore or run aspire.cli@13.5.3 (exit code 1). Run '"C:\Program Files\dotnet\dotnet.exe" exec "C:\Program Files\dotnet\sdk\10.0.303\dotnet.dll" dnx --yes aspire.cli@13.5.3 -- --version' to diagnose package sources, authentication, or connectivity.
Running the ComputeRunArguments target to discover run commands failed for this project. Fix the errors and warnings and run again.
```

Calling dnx manually
```
> &"C:\Program Files\dotnet\dotnet.exe" exec "C:\Program Files\dotnet\sdk\10.0.303\dotnet.dll" dnx --yes aspire.cli@13.5.3 -- --version
Unhandled exception: Unable to load the service index for source https://REDACTED/nuget/index.json.
```
But 13.5.3 does exist, so DNX ought to be able to use it.

```
> dir $env:NUGET_PACKAGES\aspire.cli

Directory: S:\_packages\nuget\aspire.cli

Mode LastWriteTime Length Name
---- ------------- ------ ----
d---- 26/08/2026 18:40  13.5.3
```

### Aspire doctor output

```plain
N/A
```

### Anything else?

_No response_

Reproduce with AspireUseCliBundle=true and AspireCliInvocationMode=DnxPinned, then run dotnet run while offline and inspect the DNX invocation reported in Aspire.Hosting.AppHost.targets. Done means a previously acquired aspire.cli version can be used without network connectivity, with coverage for the offline case.

Written by the indexing model from the issue text.