Enable building in-memory projects

[jjonescz]

Fixes #12058 via a global property opt-in (to avoid a breaking change).
To be used by file-based apps: dotnet/sdk#49745

[Copilot]

Pull Request Overview

This PR adds support for building in-memory or non-existent projects by opt-in via a new global property, avoiding a breaking change.

• Introduces a check for the BuildNonexistentProjectsByDefault global property to default to Build behavior
• Adds equivalent logic in the backend intrinsic MSBuild task
• Adds unit tests covering in-memory project scenarios with explicit and default behavior

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/Tasks/MSBuild.cs	Checks BuildNonexistentProjectsByDefault to set skip behavior to Build
src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/MSBuild.cs	Mirrors the same property check in the request builder
src/Tasks.UnitTests/MSBuild_Tests.cs	Adds tests for explicit Build, explicit Error, and default Build scenarios

Comments suppressed due to low confidence (3)

src/Tasks/MSBuild.cs:300

• [nitpick] The property name uses 'Nonexistent' in one word, while the enum SkipNonExistentProjectsBehavior uses 'NonExistent'. Consider aligning the casing and spelling (e.g., 'BuildNonExistentProjectsByDefault') for consistency.

                        .TryGetValue("BuildNonexistentProjectsByDefault", out var buildNonexistentProjectsByDefault) &&

src/Tasks/MSBuild.cs:299

• Add XML documentation or inline comments for the new BuildNonexistentProjectsByDefault global property in the MSBuild task, explaining its purpose and usage.

                    else if (BuildEngine is IBuildEngine6 buildEngine6 && buildEngine6.GetGlobalProperties()

src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/MSBuild.cs:333

• Add unit tests for the backend MSBuild intrinsic task to verify that BuildNonexistentProjectsByDefault correctly triggers SkipNonExistProjectsBehavior.Build in the request builder scenario.

                    else if (BuildEngine is IBuildEngine6 buildEngine6 && buildEngine6.GetGlobalProperties()

[JanProvaznik]

the property should be documented in src\MSBuild\MSBuild\Microsoft.Build.CommonTypes.xsd

please address comments, otherwise looks good

[jjonescz]

the property should be documented in src\MSBuild\MSBuild\Microsoft.Build.CommonTypes.xsd

missed this comment, looking now

[rainersigwald]

Approach, implementation, and tests look pretty good.

Could you please write up a speclet describing the change in our documentation/specs folder, from the MSBuild perspective? Or link to a larger file-based apps spec that mentions it in the PR description.