UnbindChord/GetChord/HasChordBinding(commandId) throw on a blank id once a profile is active, but return false/null otherwise #129

What's wrong

The active-profile overloads in Keybinding/Services/KeybindingService.cs don't guard commandId:

• GetChord(commandId) (~line 93)
• UnbindChord(commandId) (~line 131)
• HasChordBinding(commandId) (~line 253)

They forward to Profile.GetChord, Profile.RemoveChord and Profile.HasChord, which throw ArgumentException on whitespace. The (profileId, commandId) overloads check string.IsNullOrWhiteSpace(commandId) and return null or false.

Reproduction

svc.UnbindChord("  ");        // no active profile  -> false
svc.SetActiveProfile("p");
svc.UnbindChord("  ");        // -> throws ArgumentException
svc.GetChord("  ");           // -> throws ArgumentException
svc.GetChord("p", "  ");      // -> null

This was reproduced with a temporary MSTest test.

Why it matters

The result for the same input depends on whether a profile happens to be active, which is hidden state. The documented contract for UnbindChord is "false if it didn't exist or no active profile", and a caller that follows it can crash. The one-argument and two-argument overloads also disagree for identical inputs.

This is separate from #123. That issue is about a Command whose id has surrounding whitespace; this one is about blank ids passed to the lookup and unbind methods.

Suggested fix

Add the same if (string.IsNullOrWhiteSpace(commandId)) return null/false; guard that the profile-id overloads use.

Acceptance: for a blank commandId, the one-argument overloads return the same result as the two-argument ones, whether or not a profile is active.