fix(examples/multi_platform_seller): list_creatives response missing required query_summary

[bokelley]

Summary

Both `MockGuaranteedPlatform` and `MockNonGuaranteedPlatform` in `examples/multi_platform_seller/` fail `list_creatives` storyboard validation because the response is missing the required `query_summary` field.

Failure (from PR #508 storyboard artifact, post-fix)

```
storyboard: media_buy_seller/creative_fate_after_cancellation
step: list_creatives_before_cancel
task: list_creatives
error: mcp_error: Output validation error: 'query_summary' is a required property
```

Both tenants hit this. 1 of the 4 remaining failures per tenant after PR #508's main mock-shape fixes; the other 3 are blocked on #509 (MCP error projection).

Fix

Look at the `list_creatives_response.json` schema in `src/adcp/types/generated_poc/bundled/creative/` — `query_summary` is a required field with totals/counts of the result set. The mocks need to populate it on every list_creatives response.

Should be a 5-line fix per platform.

Acceptance

• Both mocks return `query_summary` on `list_creatives` responses
• Storyboard `creative_fate_after_cancellation` step `list_creatives_before_cancel` passes for both tenants
• Existing tests stay green

Refs

• Surfaced by PR fix(examples): multi_platform_seller passes storyboards (drops continue-on-error) #508 storyboard artifact analysis
• Fits the same "mock honesty" pattern as fix(examples): multi_platform_seller passes storyboards (drops continue-on-error) #508

[bokelley]

Triage

Classification: Bug
Bucket(s): examples
Status: drafting-pr
Milestone: (omitted — no milestone specified)

What the experts said:

• code-reviewer: approved after fixing two blockers — idempotency-replay dedup (dict-keyed upsert instead of append-only list) and explicit sandbox=False on list_creatives_response
• dx-expert: approved — pattern (import helper → store in sync → expose via list) is the right pedagogic shape; sandbox=False fix prevents production copies from shipping sandbox-flagged responses

My take: Both mocks were missing list_creatives entirely, falling back to the parent stub and producing a schema-invalid response (missing required query_summary, pagination, and creatives). Draft PR #526 adds the method to both platforms, stores approved creative items during sync_creatives using a dict keyed by creative_id for idempotency safety, and returns them via list_creatives_response. 13 new unit tests cover all acceptance criteria.

Drafting PR: add list_creatives to MockGuaranteedPlatform and MockNonGuaranteedPlatform with schema-valid query_summary — PR #526.

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Triaged by Claude Code. Session: https://claude.ai/code/session_015eWELLixehtjTexkZ3ahvr

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