feat: add assets_optional field for better asset discovery

[BaiyuScope3]

Add assets_optional field to format schema for better asset discovery

Problem

Currently, buyers and AI agents have no way to discover what optional assets a creative format supports. The assets_required array only tells them the minimum assets needed for a valid creative, but formats often support additional assets that can enhance the creative experience:

• Companion banners for video ads
• Third-party impression trackers for measurement
• End cards for video formats
• Secondary logos or background images

Without a way to advertise these capabilities, AI agents building creatives miss opportunities to create richer experiences, and buyers don't know what assets they could provide.

Solution

Add a new assets_optional field to the format schema with the same structure as assets_required. This enables:

1. Full asset discovery - Buyers/agents can see ALL assets a format accepts, not just required ones
2. Richer creatives - AI agents can include optional assets when available
3. Better tooling - Creative tools can show complete asset options to users
4. Third-party tracking support - Formats can advertise support for impression trackers

Example

{
  "format_id": { "agent_url": "https://creative.adcontextprotocol.org", "id": "video_30s" },
  "assets_required": [
    { "item_type": "individual", "asset_id": "video_file", "asset_type": "video" }
  ],
  "assets_optional": [
    { "item_type": "individual", "asset_id": "end_card", "asset_type": "image" },
    { "item_type": "individual", "asset_id": "companion_banner", "asset_type": "image" },
    { "item_type": "individual", "asset_id": "impression_tracker", "asset_type": "tracking_pixel" }
  ]
}

Changes

• Schema: Add assets_optional to format.json (same structure as assets_required)
• Server: Include assets_optional in format API responses
• Documentation: Update format docs, asset-types, implementing-creative-agents, and task references
• Version: Bump to 2.5.2

Migration

Non-breaking change. Existing formats without assets_optional continue to work unchanged. Consumers can adopt at their own pace.