tools: Add conditional import to generated projects

[jskeet]

See Librarian migration doc for details.

[gemini-code-assist]

Code Review

This pull request introduces a new style of project file augmentation in NonSourceGenerator.cs by adding a conditional import element to the generated project file. A critical issue was identified where calling doc.Add() on an XDocument will throw an InvalidOperationException because the document already has a root element. The reviewer suggested appending the Import element to the root element using doc.Root.Add() instead.

[gemini-code-assist]

Since doc is an XDocument, calling doc.Add() to add an XElement will throw an InvalidOperationException ("This document already has a root element.") because the document already has a root element (Project). The <Import> element should be added to the root element instead using doc.Root.Add().

            doc.Root.Add(new XElement("Import",
                new XAttribute("Project", importFile),
                new XAttribute("Condition", $"Exists('{importFile}')")));

[jskeet]

"Since doc is an XDocument"

From the code at line 271:

var doc = new XElement("Project", ...)

[jskeet]

Note: I was slightly surprised that the Docker container tests didn't need updating, but they don't - they're sensitive to the presence of the files, and that the generated files do actually build, but not to the specific content.

[efevans]

What's the common theme for projects that are currently relegated to ReplaceAll such that they won't be generated with Librarian? I see a couple of ConformanceTests projects and one *.CommonTesting project.

[jskeet]

All of the ReplaceAll ones are - I think - handwritten code, and I think none of them fall into the "production code, unit testing, integration testing or snippets" category which will be generated. Will need to double check though.