Migrate SyncPackageTest.ps1 PowerShell E2E tests to Apex C# tests

[Copilot]

Bug

Progress: https://github.com/nuget/client.engineering/issues/2085

Description

Migrates all 3 tests from test/EndToEnd/tests/SyncPackageTest.ps1 to a new Apex C# test class SyncPackageTestCase.cs in test/NuGet.Tests.Apex/NuGet.Tests.Apex/NuGetEndToEndTests/. The original PowerShell test file is deleted.

Migration Decision Table

PS Test Function	Decision	Apex Test Method	Notes
Test-SyncPackagesInSolutionUp	✅ Migrated	SyncPackageFromPMC_UpVersion_SyncsAllProjectsAsync	Syncs package A from the higher-version project to all other projects. Verifies both A and transitive dependency B are updated in all projects.
Test-SyncPackagesInSolutionDown	✅ Migrated	SyncPackageFromPMC_DownVersion_SyncsAllProjectsAsync	Syncs package A from the lower-version project. Verifies A is downgraded across projects; transitive dependency B is not downgraded.
Test-SyncPackagesInSolutionPlural	✅ Migrated	SyncPackageFromPMC_MultipleProjects_SyncsAllProjectsAsync	Syncs package A from a middle version (3.0.0) across 5 projects, all different starting versions. Verifies all projects converge to 3.0.0.

Approach

• Project types: ConsoleApplication (packages.config) + ClassLibrary (packages.config), matching the original New-ConsoleApplication / New-ClassLibrary PS helpers
• Package creation: CommonUtility.CreateDependenciesPackageInSourceAsync() creates packages A with a declared dependency on B at corresponding versions, replacing $context.RepositoryPath local packages
• PMC commands: nugetConsole.Execute() is used for all Install-Package (with -Version and -Source) and Sync-Package commands
• Multi-project: additional projects are added via testContext.SolutionService.AddProject()
• Assertions: CommonUtility.AssertPackageInPackagesConfig() and AssertPackageNotInPackagesConfig() replace Assert-Package and Assert-Null (Get-ProjectPackage ...)

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.

[nkolev92]

I verified the tests are passing in NuGet.Client-VS.