Use ProjectGraph for DTB

[tmat]

Remove usage of MSBuildFileSetFactory and DotNetWatch.targets for design-time build performed by dotnet-watch Hot Reload.
Replace the logic using ProjectGraph evaluation. Avoids launching additional dotnet build just to perform design-time builds.
The next step would be to switch the implementation to use Roslyn MSBuild host APIs to avoid duplicating DTB logic (both at runtime and implementation details).

Keeps the original implementation around for dotnet-watch --no-hot-reload for now.

Implements #40214
Fixes #49542
Fixes #49115

[Copilot]

Pull Request Overview

This PR updates the DTB evaluation to leverage ProjectGraph, refactoring how evaluation results are created and used throughout dotnet‐watch and its test suites.

• Updates test and production code to use MSBuildFileSetFactory.EvaluationResult instead of a custom EvaluationResult type.
• Adjusts file watching and project graph loading logic to use the project path from the current context.
• Revises test expectations and build arguments to align with the new evaluation approach.

Reviewed Changes

Copilot reviewed 17 out of 19 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/dotnet-watch.Tests/Watch/BuildEvaluatorTests.cs	Updated EvaluationResult instantiation to use MSBuildFileSetFactory.EvaluationResult.
test/dotnet-watch.Tests/HotReload/CompilationHandlerTests.cs	Refactored project graph loading to use the new static TryLoadProjectGraph method.
test/dotnet-watch.Tests/HotReload/ApplyDeltaTests.cs	Modified test expectations for hot reload output to match updated behavior.
test/dotnet-watch.Tests/Build/EvaluationTests.cs	Added comprehensive tests for project evaluation based on the new ProjectGraph usage.
test/Microsoft.NET.TestFramework/ToolsetInfo.cs	Introduced a new constant for the target framework moniker.
test/Microsoft.NET.TestFramework/ProjectConstruction/TestProject.cs	Changed file writing logic to include both source files and embedded resources.
src/BuiltInTools/dotnet-watch/Watch/MsBuildFileSetFactory.cs	Refactored EvaluationResult creation and updated MSBuild arguments.
src/BuiltInTools/dotnet-watch/Watch/BuildEvaluator.cs	Updated type references and file watching calls to use the current project context.
src/BuiltInTools/dotnet-watch/Build/EvaluationResult.cs	Reworked EvaluationResult to require a non-null ProjectGraph and added a static TryCreate method.
Other files	Various supporting updates, including changes in working directories, logger usage, and project file references.

[tmat]

@DustinCampbell ptal

[DustinCampbell]

I went through the latest updates to this PR and it still looks good to me.

✅