Add generative seller storyboard variant (media_buy_generative_seller)

[bokelley]

Summary

Need a storyboard that validates generative DSP/seller agents — platforms where creative is generated at buy time rather than uploaded separately.

Key behaviors to validate

Creative formats for generative sellers

• list_creative_formats should return formats that accept creative briefs + brand references as inputs (not just static asset specs)
• Generative sellers may ALSO support standard IAB formats (display image, HTML, JS, VAST, etc.) — a programmatic player that doesn't support these is incoherent
• Platform-type coherence: if you declare yourself a generative DSP, your format catalog should match expectations

Creative brief flow via sync_creatives

• Creative brief is passed through sync_creatives with format_ids that accept brief asset types
• The brand reference and operator context should be reflected in the response
• Validate that the agent is actually reading operator/brand from the request (e.g., echo them back, or error on invalid ones)

AgenticAdvertising.org integration

• Operator and brand should resolve via AgenticAdvertising.org registry
• Need to validate that the pull from AgenticAdvertising.org is happening (potentially by sending fake/invalid brand domains and checking for appropriate errors)

Platform-type coherence checks (general capability)

This is a broader need: validate cross-tool consistency based on declared platform type.

• "You said you're a programmatic DSP but don't support VAST"
• "You said you're a retail media network but don't support catalogs"
• "You said you're a generative seller but your formats don't accept briefs"

This could be a new validation type (check: platform_coherence) or a separate pass that runs after storyboard steps.

Relationship to comply_test_controller

The generative flow may involve async creative generation. The comply_test_controller should be able to force creative status transitions (e.g., force_creative_status: { creative_id: "...", status: "ready" }) for deterministic testing.

See also: #454 (training agent async flakiness), #453 (build-adcp-agent skill)