AdCP 3.3: wire the list_proposals and list_collections tasks (experimental) #1645

Spec

3.3.0-beta.0 adds two request/response task pairs:

• list_proposals (experimental, media_buy.open_opportunities): returns proposal snapshots beside their disposition. It's required before later_proposals.supported. It comes with opportunity.later_proposals consent, opportunity.status: "open" on accept_proposal / buy_products, and the proposal.created notification on account notification_configs[].
• list_collections (experimental, media_buy.collection_catalog / collection cards).

What's new §Open opportunities and later proposals; media-buy/task-reference/list_proposals (tag v3.3.0-beta.0).

Work (after regen)

• Task registration: ADCPHandler methods, the decisioning platform/specialism hooks, MCP tool and A2A skill registration, the spec tool list, and client methods (ADCPClient.list_proposals / list_collections), with capability-gated advertisement.
• Proposal stores can serve list_proposals from stored snapshots.
• proposal.created notification envelope support.
• Tests and reference-seller wiring if cheap.

Batch: E (experimental surfaces).