Fix COM threading issue in Office interop examples (HRESULT 0x8001010A)

[Copilot]

The Office interop code examples were causing COM exceptions due to threading and timing issues when Word was made visible during COM operations. This resulted in the error:

System.Runtime.InteropServices.COMException: The message filter indicated that the application is busy. (Exception from HRESULT: 0x8001010A)

Changes Made

Fixed the timing pattern in all Word interop methods:

• Changed wordApp.Visible = true at the beginning to wordApp.Visible = false
• Added wordApp.Visible = true at the end after all COM operations complete
• Added explanatory comments about avoiding COM threading issues

Updated methods:

• Main DisplayInWord() method in OfficeHowTo namespace
• DisplayInWord() method in Parts namespace (Snippet6)
• VS2008() method in Parts namespace

Enhanced documentation:

• Added a NOTE explaining the COM threading issue and solution approach
• Clarified why Word is kept invisible during operations

Technical Background

The HRESULT 0x8001010A error occurs when COM operations are performed on a visible Word application due to message filtering conflicts between the host application and Word's UI thread. By keeping Word invisible during operations, we eliminate the timing window where these conflicts occur.

This approach is more reliable than alternatives like Thread.Sleep() (unpredictable timing) or implementing a custom MessageFilter (overly complex for documentation examples).

Before/After Example

Before (problematic):

var wordApp = new Word.Application();
wordApp.Visible = true;  // Can cause COM exception
var doc = wordApp.Documents.Add();
// ... perform operations while visible

After (fixed):

var wordApp = new Word.Application();
wordApp.Visible = false;  // Keep invisible during operations
var doc = wordApp.Documents.Add();
// ... perform all operations safely
wordApp.Visible = true;   // Show result when complete

The fix maintains all original functionality and educational value while making the examples stable and reliable.

Fixes #18862.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/advanced-topics/interop/how-to-use-named-and-optional-arguments-in-office-programming.md	docs/csharp/advanced-topics/interop/how-to-use-named-and-optional-arguments-in-office-programming

[BillWagner]

You need to edit the snippets.5000.json file in the snippets/NamedAndOptional folder to change the error location from line 8 to line 5 (see line 6 of the json file)

[Copilot]

You need to edit the snippets.5000.json file in the snippets/NamedAndOptional folder to change the error location from line 8 to line 5 (see line 6 of the json file)

Updated the error location in snippets.5000.json from line 8 to line 5 to correctly point to the using Word = Microsoft.Office.Interop.Word; statement. Commit: 84eaaa8

[BillWagner]

Marking this ready for final review.

Tagging @adegeo to look at the expected errors and snippets 5000. This looks right to me, but that expected error is still causing a failure.