Consolidate file-level directive manipulation

[jjonescz]

dotnet project convert used custom logic to remove #: directives which didn't handle whitespace as nicely as the FileBasedAppSourceEditor (used by dotnet package add/remove). This makes both commands use the same machinery.

Furthermore, fixes some bugs in whitespace handling (discovered by the directive tests that now use the new machinery).

[jjonescz]

Moved these to VirtualProjectBuildingCommand because

• they now need to use the FileBasedAppSourceEditor which isn't available in this project,
• there is no need for these APIs to live in this shared project since they are not used by anyone but dotnet CLI (@tmat I assume you don't need these in dotnet-watch?).

[Copilot]

Pull request overview

This PR consolidates file-level directive (#:) manipulation logic by making both dotnet project convert and dotnet package add/remove use the same FileBasedAppSourceEditor machinery. The consolidation eliminates duplicate code and improves whitespace handling consistency across commands.

Changes:

• Moved directive removal logic from VirtualProjectBuilder to VirtualProjectBuildingCommand, leveraging FileBasedAppSourceEditor for consistent behavior
• Refactored WhiteSpaceInfo struct to distinguish between blank lines (BlankLineLength) and same-line whitespace (RestLength), enabling smarter whitespace removal
• Fixed whitespace handling bugs where leading/trailing blank lines weren't removed correctly in certain scenarios

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Cli/dotnet/Commands/Run/FileBasedAppSourceEditor.cs	Updated Remove method to use new BlankLineLength/RestLength properties for more precise blank line removal
src/Cli/dotnet/Commands/Run/VirtualProjectBuildingCommand.cs	Added RemoveDirectivesFromFile methods that use FileBasedAppSourceEditor for directive removal
src/Cli/dotnet/Commands/Project/Convert/ProjectConvertCommand.cs	Updated to call VirtualProjectBuildingCommand.RemoveDirectivesFromFile instead of VirtualProjectBuilder.RemoveDirectivesFromFile
src/Cli/Microsoft.DotNet.FileBasedPrograms/FileLevelDirectiveHelpers.cs	Refactored WhiteSpaceInfo to track blank lines separately from partial-line whitespace; updated whitespace calculation to exclude the directive's own span
src/Microsoft.DotNet.ProjectTools/VirtualProjectBuilder.cs	Removed RemoveDirectivesFromFile methods (consolidated into VirtualProjectBuildingCommand)
test/dotnet.Tests/CommandTests/Run/FileBasedAppSourceEditorTests.cs	Added tests for leading whitespace handling and whitespace outside lines
test/dotnet.Tests/CommandTests/Project/Convert/DotnetProjectConvertTests.cs	Updated test expectation and method call to reflect new whitespace handling behavior
src/Cli/Microsoft.DotNet.FileBasedPrograms/InternalAPI.Unshipped.txt	Updated API surface to reflect WhiteSpaceInfo property renames

[RikkiGibson]

checking my understanding: #: directives cannot have trailing // comments or similar? Is that right?

[jjonescz]

that's right, they cannot, everything on the same line as the directive is just its "content"