feat: add A2A transport binding

[maxekman]

Description

Add A2A (Agent-to-Agent) protocol as a transport for A2UI, enabling agents
to be served and consumed over HTTP via the A2A protocol.

• A2UI.A2A — server-side macro wrapping an A2UI agent as an A2A agent
• A2UI.Transport.A2A — client-side transport connecting LiveViews to remote agents
• A2UI.A2A.Delivery — minimal transport for agent→handler message delivery
• Sync fence ({:a2ui_sync, pid, ref}) in A2UI.Agent for reliable message collection
• Catalog validation now accepts custom component_modules types
• Demo page at /a2a showing the same agent over A2A transport

Steps to reproduce

mix test    # 400 tests, 0 failures
mix a2ui.demo
# http://localhost:4002     → Local transport
# http://localhost:4002/a2a → A2A transport

Environment

• Elixir: ~> 1.17
• OTP: 27
• a2ui version: 0.1.0

[claude]

Reviewed — one high-confidence issue found.

Unhandled {:a2ui_transport_error, reason} crashes the LiveView on any A2A network error

A2UI.Transport.A2A sends {:a2ui_transport_error, reason} to the LiveView on HTTP failures (both in init_handler/3 and handler_loop/3). use A2UI.Live only injects a handle_info for {:a2ui_message, msg}, so this message is unhandled, causing a FunctionClauseError and a LiveView crash. The demo A2ALive also has no clause for it.

Fix options:

• Add a handle_info({:a2ui_transport_error, reason}, socket) clause to the use A2UI.Live macro (log + noreply), or
• Have the transport deliver errors as a typed {:a2ui_message, ...} so the existing handler picks them up.

[maxekman]

Fixed — added handle_info({:a2ui_transport_error, reason}, socket) to the use A2UI.Live macro. Logs the error and returns {:noreply, socket}, matching the pattern of all other error handlers in the module.