refactor: make session pickers individual action items (#299564)

Instead of bundling all extension-contributed session pickers into a
single container action item (ChatSessionPickersContainerActionItem),
each picker is now its own individual action item in the toolbar.

This matches the pattern of built-in pickers (model, mode, session
target) where each picker gets its own <li> element. The toolbar's
responsive overflow logic can now independently shrink/hide each picker.

Changes:
- Dynamic menu item registration in ChatSessionsService when option
  groups are set via setOptionGroupsForSessionType()
- Per-group visibility context keys (chatSessionPickerGroup.<groupId>)
  for controlling individual picker visibility via when clauses
- Updated actionViewItemProvider to handle dynamic action IDs
- Changed CSS class from chat-sessionPicker-item to chat-input-picker-item
  (same class used by built-in pickers)
- Removed ChatSessionPrimaryPickerAction and
  ChatSessionPickersContainerActionItem

Fixes: #299564

Co-authored-by: Copilot <[email protected]>