Added the .NET side of the document open endpoint

app/MindWork AI Studio/Tools/Rust/OpenDocumentRequest.cs

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+namespace AIStudio.Tools.Rust;
+
+/// <summary>
+/// Asks the runtime to open a document in the program the system uses for it.
+/// </summary>
+/// <param name="Path">The document to open.</param>
+/// <param name="Page">The page to show, counted from one, or null when the document has none.</param>
+public readonly record struct OpenDocumentRequest(string Path, int? Page);

app/MindWork AI Studio/Tools/Rust/OpenDocumentResponse.cs

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+namespace AIStudio.Tools.Rust;
+
+/// <summary>
+/// Says how opening a document went.
+/// </summary>
+/// <param name="Success">Whether the document was opened at all.</param>
+/// <param name="PageApplied">
+/// Whether the document was handed to its program together with the page. False means it opens on
+/// its first page: no page was asked for, the system uses a program which cannot be told one, or
+/// starting that program failed. None of these is an error, so this belongs in the log rather than
+/// in front of the user, who is told the page by the source itself.
+/// </param>
+/// <param name="Issue">Why the document could not be opened, or an empty text when it was.</param>
+public readonly record struct OpenDocumentResponse(bool Success, bool PageApplied, string Issue);