[Repo Assist] Fix: respect (IsTestProject)false(/IsTestProject) MSBuild property to exclude projects from Test Explorer

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Summary

Closes #1970

Users who have projects that reference test SDK packages (via shared paket dependencies or otherwise) but explicitly set (IsTestProject)false(/IsTestProject) in their project file still see those projects in the Test Explorer and receive "no tests discovered" warnings. This PR fixes that.

Root Cause

ProjectExt.isTestProject in TestExplorer.fs determines whether a project should appear in the Test Explorer by checking if its package references include Microsoft.TestPlatform.TestHost or Microsoft.NET.Test.Sdk. This check ignores the IsTestProject MSBuild property, which is the authoritative way in .NET to declare whether a project is a test project.

The Project DTO already exposes IsTestProject: bool via project.Info.IsTestProject (populated from FsAutoComplete's ProjectResponseInfoDotnetSdk). This property correctly reflects MSBuild's computed value: it is set to true by the Microsoft.NET.Test.Sdk package targets when the SDK is present, and overridden to false when the user adds (IsTestProject)false(/IsTestProject) to their project file.

Fix

Replace the manual package-reference scan with a direct check of project.Info.IsTestProject:

// Before:
let isTestProject (project: Project) =
    let testProjectIndicators =
        set [ "Microsoft.TestPlatform.TestHost"; "Microsoft.NET.Test.Sdk" ]
    project.PackageReferences
    |> Array.exists (fun pr -> Set.contains pr.Name testProjectIndicators)

// After:
let isTestProject (project: Project) =
    project.Info.IsTestProject

This is semantically equivalent for standard projects (the Microsoft.NET.Test.Sdk package sets IsTestProject=true in its .targets file), while correctly respecting explicit user overrides.

Trade-offs

• Standard projects (xUnit, NUnit, MSTest): No behaviour change — these all include Microsoft.NET.Test.Sdk, which sets IsTestProject=true.
• Expecto projects: Typically include Microsoft.NET.Test.Sdk transitively (or directly), so IsTestProject=true. If an Expecto project does not use Test SDK, IsTestProject may be false and would no longer appear in Test Explorer. This is actually correct — without Test SDK, dotnet test --list-tests wouldn't work on it anyway.
• Projects with explicit (IsTestProject)false(/IsTestProject): Now correctly excluded from Test Explorer. This is the fix for MSBuild <IsTestProject> is ignored #1970.
• Removing the manual package list: No more need to update this list if new test runners introduce different SDK package names.

Test Status

⚠️ Infrastructure limitation: dotnet paket restore and yarn install are unavailable in this environment due to network restrictions, so a full build cannot be run.

The change is a one-line simplification from a multi-line array scan to a single property access. The IsTestProject field is of type bool and is already used elsewhere in TestExplorer.fs (e.g., project.Info.TargetFramework). The change is syntactically and type-correct.

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