[BUG] existingDotnetPath setting is ignored if the tool autodetects preinstalled runtime

Describe the Issue

Like described in #314 or #138 I put the existingDotnetPath setting in my settings.json, but the tool detects a dotnet somewhere else, and uses that. This is an issue for me because this other dotnet doesn't have the correct SDKs installed.

Starting Spawn .NET server...
Starting Open a solution...
Starting Open a solution with environment service...
Starting Clear environment...
Using preinstalled .NET runtime at "C:\Program Files\dotnet\dotnet.exe"
.NET server started and IPC established in 2812ms

Steps To Reproduce

settings.json:

{
    "dotnetAcquisitionExtension.existingDotnetPath": [
        {
            "extensionId": "ms-dotnettools.csdevkit",
            "path": "C:\\Users\\User\\AppData\\Local\\Microsoft\\dotnet\\dotnet.exe"
        },
        //ms-dotnettools.vscodeintellicode-csharp, ms-dotnettools.csharp, ms-dotnettools.vscode-dotnet-runtime
    ]
}

Expected Behavior

Either the option existingDotnetPath should take precedence, or there needs to be a way to make it stop autodetecting preinstalled SDKs and force a download.

Environment Information

VS Code: 1.84.2 (user setup)
C# extension: v2.12.19
C# Dev Kit extension: v1.0.14
.NET Install Tool: v2.0.0

Start with the settings.json example and the runtime startup log showing the preinstalled path. Trace how existingDotnetPath is handled relative to preinstalled runtime autodetection, then verify that the configured path is honored or that force-download behavior is available. Done means the reported configuration no longer selects the wrong runtime and the behavior is covered by a regression test.