C#: Fix AutoFormat corrupting whitespace outside target subtree

[knutwannheden]

Motivation

When CSharpTemplate.Rewrite() or AutoFormatVisitor formats a specific subtree (e.g., replacing [Fact] with [Test]), the RoslynFormatter.Format() method corrupts whitespace in unrelated parts of the file. For example:

• Base type lists: IComparable<TestObj?>, IEquatable<TestObj?> → IComparable<TestObj?> , IEquatable<TestObj?>
• Async modifiers: async Task<RunSummary> → async Task<RunSummary>

This happens because MinimumViableSpacingVisitor (MVS) adds spacing artifacts to ensure parseable output, and when reconciling whitespace back, those artifacts leak into nodes outside the formatting target.

Summary

• Root cause: Format() overwrote cu with the MVS result, so whitespace reconciliation used the MVS-modified tree as the "original" — leaking MVS artifacts into non-target nodes
• Fix: Save originalCu before MVS, use mvsCu only for printing/Roslyn formatting, reconcile against originalCu. This matches the pattern already used in FormatSpans()
• Changed file: RoslynFormatter.cs — the Format(cu, targetSubtree, stopAfter) overload

Test plan

• Added FormatSubtreeDoesNotCorruptUnrelatedWhitespace test with base type lists (nullable generics) and async methods
• Test exercises both FormatSubtree and Format code paths
• Asserts full character-level equality (not just substring checks)
• All 20 AutoFormatTests pass
• All 50 format-related tests pass