fix(examples/multi_platform_seller): list_creatives populates query_summary

[bokelley]

Summary

Closes #510. Surfaced by PR #508's storyboard artifact: post-#508, both tenants still fail one storyboard step — creative_fate_after_cancellation/list_creatives_before_cancel — because the mocks didn't implement list_creatives at all, and the wire contract requires query_summary on every response.

This PR adds list_creatives to both MockGuaranteedPlatform and MockNonGuaranteedPlatform. Each mock now:

• Persists creatives at sync_creatives time into an in-memory library (_creatives: dict[str, dict] keyed by creative_id)
• Returns the full library on list_creatives with the required query_summary block (total_matching, returned) and pagination block (has_more=False, total_count)
• Projects each item to the schemas/3.0.6/creative/list-creatives-response.json Creative shape: {creative_id, name, format_id, status: "approved", created_date, updated_date}

Auto-approval (status: "approved") mirrors the existing sync_creatives policy on both mocks. Pagination is intentionally trivial — the storyboard catalog is small and modeling cursor-based paging would add noise without illustrating a new platform-shape concern.

Expected delta

Per-tenant pass count should go up by 1 (one storyboard step now passes per tenant). The remaining 3 failures per tenant are blocked on #509 (MCP error projection), per the issue.

Test plan

• ruff check examples/multi_platform_seller/ — clean
• mypy src/adcp/ — clean (777 source files)
• pytest tests/ -x -q — 3760 passed, 32 skipped, 1 xfailed
• Functional sanity: ListCreativesResponse.model_validate(...) accepts the output on both mocks across empty-library and populated-library cases
• CI storyboard run on tenant-a + tenant-b — verifies list_creatives_before_cancel now passes

Refs

• Closes fix(examples/multi_platform_seller): list_creatives response missing required query_summary #510
• Surfacing context: PR fix(examples): multi_platform_seller passes storyboards (drops continue-on-error) #508 storyboard artifact, post-fix
• Sibling open issue: feat(server): MCP error responses must populate structuredContent.adcp_error.code (not just text) #509 (MCP error projection — out of scope here)

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