Phase 2: Push-notification config persistence hook on create_a2a_server (PR-O)

[bokelley]

Summary

a2a-sdk's DefaultRequestHandler accepts a push-notification-config store, but adcp.server.create_a2a_server() doesn't surface it. Downstream (salesagent) has 8 `on_*_push_notification_config` methods backed by a DB table; adopting the SDK's A2A path drops all of that on the floor.

Proposed API

def create_a2a_server(
    handler: ADCPHandler,
    *,
    ...,
    push_notification_config_store: PushNotificationConfigStore | None = None,
) -> Starlette: ...

Mirror a2a-sdk's store ABC — or expose our own thin ABC that wraps theirs — and thread it into DefaultRequestHandler(push_notification_config_store=...). Default to the in-memory impl for demo use; document the persistence implications clearly.

Acceptance

• Kwarg accepted on create_a2a_server.
• Integration test exercising set/get/list/delete of push-notif configs with a file-backed store.
• Reference example alongside Phase 2: Pluggable TaskStore on create_a2a_server (PR-N) #224's DB-backed TaskStore.

Context

Roadmap item PR-O (Phase 2) from .context/sdk-adoption-roadmap.md. Second of three blockers gating salesagent's A2A migration (with #224, #226).