Setting backend_type in QNN has no effect.

# Describe the bug
When loading an ONNX model with QNN, setting `backend_type` has no effect.

# To Reproduce
```csharp
var config = new Microsoft.ML.OnnxRuntimeGenAI.Config(ModelDir);
config.ClearProviders();
config.AppendProvider("QNNExecutionProvider");
config.SetProviderOption("QNNExecutionProvider", "backend_type", "gpu");
config.SetProviderOption("QNNExecutionProvider", "profiling_file_path", "C:\\ProgramData\\Dynabook\\Dynabook Assistant\\Models\\test.csv");
//var path = Path.Combine(AppContext.BaseDirectory, "QnnGpu.dll");
//config.SetProviderOption("qnn", "backend_path", path);
//config.SetProviderOption("qnn", "htp_performance_mode", "high_performance");
_model = new OnnxRuntimeGenAIChatClient(config, true, options);
```

# Expected behavior
GPU execution should take effect.

# Screenshots

# Desktop (please complete the following information)
- OS: Windows 11 24H2 26100.4946
- Microsoft.ML.OnnxRuntimeGenAI.QNN: 0.9.0
-

# Additional context
Add any other context about the problem here.

Reproduce the issue with the shown Config, SetProviderOption, and OnnxRuntimeGenAIChatClient entry points. Then trace how QNNExecutionProvider options reach the provider, focusing on backend_type and the expected GPU execution behavior. Done means setting backend_type to gpu changes execution as expected, with a regression check if the relevant test location can be identified.

Written by the indexing model from the issue text.