Walk Track: Extend test_mandatory_field refactor to remaining knife commands

[nikhil2611]

Context

Walk Ex3 replaced duplicate inline nil-guard blocks in node_show.rb, client_show.rb, and role_show.rb with calls to the existing test_mandatory_field helper. At least 15 other knife commands still use the inline pattern.

Code Paths

• lib/chef/knife/node_edit.rb:43
• lib/chef/knife/role_create.rb:42
• lib/chef/knife/client_edit.rb:36
• lib/chef/knife/cookbook_download.rb:60
• lib/chef/knife/environment_show.rb:38
• lib/chef/knife/ssh.rb:612
• lib/chef/knife/tag_list.rb:38
• lib/chef/knife/role_delete.rb:37
• lib/chef/knife/user_reregister.rb:41
• (and ~6 more — grep -rn 'ui.fatal.*must specify' lib/)

Acceptance Criteria

• All inline if @name_args[0].nil? ... ui.fatal ... exit 1 blocks in lib/chef/knife/*.rb replaced with test_mandatory_field
• No change in behavior — existing specs remain green
• Net line reduction ≥ 30 lines

Dependencies

None — safe to start immediately.

Size

Medium — one PR, no logic changes, pure refactor.