Support variables in #:project directives

[jjonescz]

Closes #49286.

[Copilot]

Pull Request Overview

Adds support for MSBuild variable expansion inside #:project directives and updates tests and documentation accordingly.

• Introduces directive evaluation pass (EvaluateDirectives) to expand MSBuild variables and resolve project paths.
• Extends CSharpDirective.Project to preserve original and unresolved names for later path adjustments during project conversion.
• Adds tests covering variable usage and updates documentation about variable handling limitations.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
RunTelemetryTests.cs	Adapts tests to new CSharpDirective.Project constructor.
RunFileTests.cs	Adds tests for variable-based project references and malformed variable syntax.
DotnetProjectConvertTests.cs	Adds test cases for variable-containing paths; updates expectations for cross-platform separators.
VirtualProjectBuildingCommand.cs	Adds directive evaluation, caching of source file, and enhanced project directive handling.
ProjectConvertCommand.cs	Integrates directive evaluation and updates path rewrite logic to preserve variable intent.
dotnet-run-file.md	Documents variable support and caveats for #:project directives.

[RikkiGibson]

The linked issue has milestone 10.0.2xx, was this PR meant to target release/10.0.2xx branch?

[jjonescz]

Definitely, I was worried there might be changes missing in the 2xx branch, but looks like it's fairly up to date wrt file-based app PRs, so I can retarget now.