Adds /a2ui-catalog page and a runtime endpoint with NO a2ui config, so
A2UI switches on purely from the provider's a2ui.catalog (the #5774
path). Also fixes DemoButtonAgent, which never actually rendered: it
emitted the wrong activity content key (operations -> a2ui_operations)
and a non-canonical operation/component format. Rewritten to the A2UI
v0.9 wire format (createSurface/updateComponents, flat components, root
id "root") so the surface paints and the Confirm round-trip works.