Model load fails inside Windows AppContainer in 1.1.0 (worked in 0.8.2.1)

# `model.LoadAsync()` fails with "Access is denied" inside a Windows AppContainer (1.1.0)

## Summary

Calling `model.LoadAsync()` from a process running inside a Windows AppContainer fails with an access-denied error originating from ONNX Runtime's `WeaklyCanonicalPath` external-data validation. The same code, same model, and same on-disk paths work when the process runs outside an AppContainer. This worked in 0.8.2.1.

## Environment

- `Microsoft.AI.Foundry.Local` **1.1.0**
- `Microsoft.AI.Foundry.Local.Core` **1.1.0**
- .NET 8.0, Windows 11
- Model: `qwen2.5-1.5b-instruct-generic-cpu:4`
- Last known working version: **0.8.2.1**

## Repro

1. Host the Foundry Local managed SDK in a process launched as a Windows AppContainer.
2. `await FoundryLocalManager.CreateAsync(config, logger);`
3. `await model.DownloadAsync();` — succeeds (~1.83 GB written to cache).
4. `await model.LoadAsync();` — **fails**.

## Error

```
Microsoft.ML.OnnxRuntimeGenAI.OnnxRuntimeGenAIException: External data path validation failed
for initializer: model.embed_tokens.weight. Error: tensorprotoutils.cc:377
onnxruntime::utils::WeaklyCanonicalPath Failed to get the weakly canonical path:
"C:\ModelRelayFoundryLocal\cache\models\Microsoft\qwen2.5-1.5b-instruct-generic-cpu-4\v4" - Access is denied.

at Microsoft.ML.OnnxRuntimeGenAI.Result.VerifySuccess(IntPtr)
at Microsoft.ML.OnnxRuntimeGenAI.Model..ctor(Config)
at Microsoft.Neutron.OpenAI.Provider.OnnxLoadedModel..ctor(String, Config, GenAIConfig, InferenceModel, OnnxEP)
at Microsoft.AI.Foundry.Local.ModelManager.LoadModelAsync(...)
at Microsoft.AI.Foundry.Local.NativeInterop.ExecuteCommandManaged(...)
at Microsoft.AI.Foundry.Local.Detail.ModelLoadManager.LoadAsync(String modelId, ...)
at Microsoft.AI.Foundry.Local.ModelVariant.LoadAsync(...)
at Microsoft.AI.Foundry.Local.Model.LoadAsync(...)
```

The path being validated is the cached model file under `AppDataDir/cache/models/...`. The file exists and is readable.

## What we tried

All produce the same error:

| `AppDataDir` value | Result |
|---|---|
| (default) | fail |
| `%LOCALAPPDATA%\ModelRelayFoundryLocal` | fail |
| `C:\.ModelRelayFoundryLocal` | fail |
| `C:\ModelRelayFoundryLocal` (no leading dot) | fail |
| The AppContainer's own per-package storage under `Packages\\AC\` | fail |

We also granted the AppContainer SID `readWrite` on `C:\` (entire drive). Same error. So this is not a filesystem ACL or path-string issue from our side.

Switching the host process to run **without** an AppContainer (no other code changes) — `LoadAsync()` succeeds and the endpoint serves requests normally.

Begin at Model.LoadAsync, ModelManager.LoadModelAsync, and the ONNX Runtime GenAI Model construction shown in the stack trace; compare the 1.1.0 behavior with 0.8.2.1 under a Windows AppContainer. Done means model.LoadAsync succeeds in the reported AppContainer setup without weakening the stated path validation, with coverage for the regression.

Written by the indexing model from the issue text.