Fix interactive TFM selection to evaluate property references in file-based app TargetFrameworks

[Copilot]

GetTargetFrameworksFromSourceFile was reading the raw TargetFrameworks directive value without expanding MSBuild property references, causing $(MyProperty) to appear as a literal framework name in the TFM selection UI/error output.

For example, this file-based app:

#:property TargetFramework=
#:property MyTfm=net9.0
#:property TargetFrameworks=$(MyTfm);net10.0
Console.WriteLine("Hello Multi-TFM");

Previously surfaced $(MyTfm) as a framework option instead of net9.0.

Changes

• RunCommand.cs: Collect all #:property directives into a dictionary before evaluating TargetFrameworks, then expand $(PropertyName) references using a new ExpandMSBuildProperties helper. Last definition wins (matching MSBuild semantics). Circular references are resolved to empty string.
• RunCommand.cs: Added [GeneratedRegex] for $(...) pattern matching (AOT-compatible); made RunCommand partial to support it.
• RunFileTests.cs: Added regression test verifying that $(MyTfm) is evaluated to its property value in the TFM selection error output.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Interactive TFM selection doesn't evaluate TargetFrameworks extracted from file-based apps</issue_title>
<issue_description>The interactive TFM selection added in #51509 (cc @jonathanpeppers) extracts TargetFrameworks property without evaluating it and hence it won't work for files like the following:

#:property TargetFramework=
#:property MyTfm=net9.0
#:property TargetFrameworks=$(MyTfm);net10.0
Console.WriteLine("Hello Multi-TFM");

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Interactive TFM selection doesn't evaluate TargetFrameworks extracted from file-based apps #52271

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