Add unit tests for CompleteWorkspaceMembers fallback chain

[rbansal42]

Context

From code review of completion follow-up PR.

Problem

CompleteWorkspaceMembers has a 3-level fallback chain (Nickname → Username → DisplayName) in internal/cmdutil/completion.go:251-257, but there are no tests exercising this logic.

Suggested Fix

Extract the name-resolution logic into a pure function like resolveMemberName(user *api.User) string and add table-driven tests covering:

• All fields present (should use Nickname)
• Nickname empty (should use Username)
• Nickname and Username empty (should use DisplayName)
• All empty (should skip the member)

Files

• internal/cmdutil/completion.go
• internal/cmdutil/completion_test.go