Wire up model variant selection instead of hardcoded SqueezeNet path

[yeelam-gordon]

Fixes ADO #61791040

The cs-wpf, cs-winui, and cs-winforms samples had hardcoded model paths ignoring the variant selection UI. This wires up the combo box selection to actually load the chosen model variant.

[Copilot]

Pull request overview

This PR fixes the WindowsML UI samples (WPF, WinUI 3, WinForms) so they no longer hardcode SqueezeNet.onnx and instead allow ModelManager.ResolvePaths() / DetermineModelVariant() to select the appropriate SqueezeNet model variant based on the selected execution provider/device configuration (aligning behavior with the UI selections).

Changes:

• Replaced hardcoded ModelPath = "SqueezeNet.onnx" with an empty ModelPath so shared model variant selection logic is used.
• Added inline comments explaining why ModelPath must be empty for variant auto-selection to work.
• Applied the same fix consistently across WPF, WinUI, and WinForms sample apps.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Samples/WindowsML/cs-wpf/MainWindow.xaml.cs	Stops hardcoding the SqueezeNet model filename so shared variant selection can choose the correct model.
Samples/WindowsML/cs-winui/MainWindow.xaml.cs	Same change for WinUI: clears ModelPath to enable shared model variant selection behavior.
Samples/WindowsML/cs-winforms/MainForm.cs	Same change for WinForms: clears ModelPath to avoid bypassing shared model variant selection.

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[nieubank]

Thanks for putting this together. During triage for ADO 61791040, we could not reproduce the reported issue as a broken hardcoded model path in the current samples: the WinUI/WPF/WinForms samples use the app-local relative model name SqueezeNet.onnx, and the shared/console paths support custom model paths via --model.

This PR adds model-variant selection behavior, which looks like a sample UX enhancement rather than a fix for the reported hardcoded-path issue.

Recommendation: close/defer this PR for ADO 61791040. If the team wants model-picker or model-variant UX work, it should be tracked separately from this bug bash item.