sessions: test the Session Activity Bar Terminal entry registration #343318

Follow-up to #343313, addressing its review comments. Tests only, no product change.

• Adds a test in sessionsTerminalContribution.test.ts that reads the registered Menus.SessionActivityBar items and checks that the Terminal entry comes first in the panel group (2_panel, order 0), before the Show Panel toggle (order 1). It also checks when the entry shows and when it's enabled: shown when a terminal can be launched or one already exists, including with layout actions off; hidden when no terminal is available and in auxiliary windows; disabled while a custom view is shown.


• The activity bar fixtures and sessionActivityBar.test.ts now use SessionActivityBarGroup.SidePanel / .Panel instead of hardcoded group ids.

Validation

• 
  npm run typecheck-client: passed


• 
  npx eslint <changed files>: passed


• 
  ./scripts/test.sh --run src/vs/sessions/contrib/terminal/test/browser/sessionsTerminalContribution.test.ts --run src/vs/sessions/test/browser/sessionActivityBar.test.ts --run src/vs/sessions/test/browser/layoutActions.test.ts: 121 passing