Add automated XML documentation validation test for FSharp.Core .fsi files

[Copilot]

This PR adds an automated test to validate that all XML documentation comments in FSharp.Core signature files are well-formed and valid XML. The test ensures documentation quality and prevents malformed XML from being introduced.

What this adds

• New test module: FSharp.Core.UnitTests.XmlDocumentationValidation that scans all .fsi files in the FSharp.Core directory
• XML validation: Uses System.Xml to parse and validate each XML documentation block for well-formedness
• Comprehensive coverage: Currently validates 2,153 XML documentation blocks across 31 .fsi files
• Detailed error reporting: Provides specific file names, line numbers, and XML parsing error messages when validation fails

How it works

The test extracts XML documentation comments (lines starting with ///) from each .fsi file, groups them into logical blocks, and validates each block by:

1. Wrapping the XML content in a root element to create a valid XML document
2. Attempting to parse it with XmlDocument.LoadXml()
3. Reporting any XmlException with precise location information

Testing and demonstration

The implementation has been verified to:

• ✅ Pass when all XML documentation is well-formed (current state)
• ✅ Fail with detailed error messages when malformed XML is introduced
• ✅ Integrate properly with the existing FSharp.Core.UnitTests test suite

The test's effectiveness was demonstrated by temporarily introducing an intentional XML documentation error with crossing tags, which the validation correctly caught with detailed error messages. The error was then reverted to maintain clean code.

This ensures that any future changes to XML documentation in FSharp.Core .fsi files will be automatically validated for correctness, preventing documentation build issues and maintaining high documentation quality standards.

This pull request was created as a result of the following prompt from Copilot chat.

Add an automated validation test for XML documentation comments in FSharp.Core .fsi files. Ensure all XML comments are well-formed and valid, and fail the test on any parse errors or invalid XML structure.

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[T-Gro]

Now make a dummy change somewhere in FSharp.Core .fsi file that will show how this new test can fail.

I want to see it fail.

E.g. make a XML invalid crossing like this:

/// <sumary>
/// Text
/// <remarks>
/// Text
/// </summary>
/// more text
/// </remarks>

[Copilot]

Now make a dummy change somewhere in FSharp.Core .fsi file that will show how this new test can fail.

I want to see it fail.
...

Done! I've introduced an intentional XML documentation error in option.fsi with crossing tags as requested. The malformed XML has an <example> tag inside a <remarks> tag, but closes the </remarks> before closing </example>, creating invalid nested XML.

When I tested the validation, it correctly failed with the error:

ERROR: Line 55: Invalid XML - The 'example' start tag on line 8 position 2 does not match the end tag of 'remarks'. Line 10, position 3.

The test successfully demonstrates how the XML documentation validation catches malformed XML and provides precise error location information. Commit: c8f0f09

[github-actions]

✅ No release notes required

[T-Gro]

@copilot : Now manually ammend the commit to show me (T-Gro) as the author, not you.