GetCommandsByCategory(null) returns nothing, while "" and " " return the uncategorized commands #140

What's wrong

CommandRegistry.GetCommandsByCategory (Keybinding/Services/CommandRegistry.cs, ~line 60):

string? normalizedCategory = category?.Trim();
.Where(c => string.Equals(c.Category, normalizedCategory, StringComparison.OrdinalIgnoreCase))

c.Category is a CommandCategory?, which is a SemanticString. Passing it to string.Equals(string, string, …) goes through the implicit string conversion, and a null CommandCategory converts to "", not null. That gives three different results:

• category: null: compares "" with null, so nothing ever matches.
• category: "" or " ": trims to "", so it matches every uncategorized command.

Repro

Register two commands:

• new Command("file.save", "Save"), with no category
• new Command("edit.copy", "Copy", null, "Edit")

Then call GetCommandsByCategory with each input:

null   -> []
""     -> [file.save]
"  "   -> [file.save]
"Edit" -> [edit.copy]

The parameter is declared string? category, so null is an intended input, and "commands with no category" is the natural meaning. A UI that groups commands by category and passes command.Category back in gets an empty "Uncategorized" group.

Suggested fix / acceptance criteria

Handle null explicitly instead of relying on the implicit conversion:

string? wanted = string.IsNullOrWhiteSpace(category) ? null : category.Trim();
.Where(c => wanted is null
    ? c.Category is null
    : c.Category is not null && string.Equals(c.Category.ToString(), wanted, StringComparison.OrdinalIgnoreCase))

• Null, "" and whitespace all return the uncategorized commands, and no others.
• Add tests for null, "", " " and "Edit".