Describe the bug

Found by @williammartin.

gh pr edit --add-assignee drops any current assignees on the issue. Likely only a problem on GitHub.com and not on GHES due to Actor assignee changes around making Copilot assignable.

Affected version

gh version 2.74.0 (2025-05-29)
https://github.com/cli/cli/releases/tag/v2.74.0

Steps to reproduce the behavior

1. Have issue with foo assigned.
2. gh pr edit 1234 --add-assignee bar
3. foo gets unassigned and bar gets assigned.

Expected vs actual behavior

Current assignees must be preserved.