Fix interaction of "csc only after msbuild" optimization and implicit build files

[jjonescz]

The added test would fail without this fix. The first commit is a minimal fix, the second commit is just improving code a bit, the third commit optimizes the check futher.

[Copilot]

Pull request overview

This PR fixes a bug in the "CSC only after MSBuild" optimization where implicit build files (like Directory.Build.props) were not always checked before deciding whether to use CSC-only builds. The fix ensures that implicit build file changes are detected even when the source file is also modified.

Key Changes:

• Refactored the CSC arguments reusability check into a dedicated helper function GetReasonToNotReuseCscArguments()
• Reordered the checks in NeedsToBuild() to always verify implicit build files before making CSC-only build decisions
• Added comprehensive test coverage for the interaction between the optimization and implicit build files

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
test/dotnet.Tests/CommandTests/Run/RunFileTests.cs	Adds new test CscOnly_AfterMSBuild_DirectoryBuildProps that verifies implicit build file changes (Directory.Build.props) are detected regardless of whether the source file is also modified, using CombinatorialData to test all scenarios
src/Cli/dotnet/Commands/Run/VirtualProjectBuildingCommand.cs	Refactors CSC argument reusability checks into GetReasonToNotReuseCscArguments() helper, reorders source file modification checks to ensure implicit build files are always checked, and simplifies GetBuildLevel() by removing duplicated validation logic

The implementation is clean and correct. The fix addresses the issue described in the PR by ensuring implicit build files are checked before the optimization decision, and the refactoring improves code maintainability by extracting the validation logic into a well-named helper function.

[jjonescz]

@333fred @RikkiGibson for more reviews, thanks