Preserve indentation in XML doc comment CDATA sections

[HominoITea]

Fixes #27150

Summary

XML documentation formatting currently applies member indentation to every physical /// line, including content inside a CDATA section. Since whitespace inside CDATA is character data, this changes the authored indentation of code samples.

This change:

• tracks CDATA boundaries while formatting single-line (///) documentation comments;
• avoids adding compiler-generated indentation to CDATA content;
• preserves the existing formatting behavior outside CDATA sections;
• adds a regression test that compares the exact emitted XML documentation text.

Testing

• CDataInCodeElementPreservesAuthoredIndentation
• DocumentationCommentCompilerTests on net10.0
• Microsoft.CodeAnalysis.CSharp.Symbol.UnitTests on net10.0
• Compiler test suite on CoreCLR via Compilers.slnf
• Compilers.slnf build on Linux

Microsoft Reviewers: Open in CodeFlow

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[HominoITea]

@dotnet-policy-service agree