sessions: enable Search Editor and Problems panel in Agents window (#314599)

- Enable Search Editor contribution in the Agents window
- Extract search.searchEditor.* configuration properties from
  search.contribution.ts into searchEditor.contribution.ts
- Create search.common.contribution.ts for shared service registrations
  (ISearchHistoryService, IReplaceService, INotebookSearchService)
- Add Cmd+Shift+F keybinding to open Search Editor in Agents window
- Enable Problems panel (markers) with WindowEnablement.Both
- Fix panel part badge positioning in Agents window CSS

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>