Style config toolbar to match session picker visuals

Updated CSS to target .sessions-chat-config-toolbar (the MenuWorkbenchToolBar
container for mode + model pickers) instead of .sessions-chat-model-picker.
Styles match .sessions-chat-picker-slot for consistent look across all pickers.

Also styles the .monaco-toolbar and .action-item within the config toolbar
to align properly.

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