fix(examples/multi_platform_seller): add list_creatives + query_summary to both mock platforms

[bokelley]

Closes #510

Summary

Both MockGuaranteedPlatform and MockNonGuaranteedPlatform were missing a list_creatives method, causing storyboard validation to fail with 'query_summary' is a required property. The list-creatives-response.json schema requires each creative item to carry creative_id, name, format_id, status, created_date, and updated_date.

Changes:

• Add self._synced_creatives: dict[str, dict] to both __init__ (keyed by creative_id so idempotency-replay re-syncs upsert rather than accumulate duplicates)
• Restructure sync_creatives in both mocks to store the creative items (with all required list-response fields) inside the same lock block as buy-status mutation
• Add list_creatives method to both mocks using list_creatives_response(values, sandbox=False), which auto-fills created_date/updated_date and builds the required query_summary/pagination envelope
• Non-guaranteed mock preserves its double-transition (pending_creatives → pending_start → active)

What was tested

• ruff check src/ tests/test_multi_platform_seller.py — clean
• mypy src/adcp/ — clean (777 source files)
• pytest tests/ -q --ignore=tests/conformance/signing/test_ip_pinned_transport.py — 3749 passed, 0 new failures (the ignored test is a pre-existing sandbox TLS failure unrelated to this change)
• 13 new unit tests in tests/test_multi_platform_seller.py covering: empty-list query_summary/pagination shape, synced-creative round-trip, all required fields present, name fallback, dedup on re-sync of same creative_id, sandbox=False in response

Pre-PR review:

• code-reviewer: approved — blockers (idempotency dedup, sandbox=False) addressed before PR; nits (module-level _DEFAULT_FORMAT_ID constant) noted below
• dx-expert: approved — pattern (import helper → store in sync → read in list) is the right mental model for adopters; sandbox=False explicit per blocker fix

Known nits (not fixed here to keep scope tight):

• The format_id fallback dict {"agent_url": "https://creative.adcontextprotocol.org/", "id": "display_300x250"} is inlined in both mock files rather than extracted to a module-level constant. Acceptable for example code; extracting it is a cosmetic follow-up.
• _synced_creatives is not filtered by account (the storyboard uses single-tenant instances, so cross-contamination can't occur in the current topology; comment noting this added inline)

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See adcp#3121
for context.

Session: https://claude.ai/code/session_015eWELLixehtjTexkZ3ahvr

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[bokelley]

Superseded by #521 (fix(examples/multi_platform_seller): list_creatives populates query_summary), already merged to main.

[bokelley]

Acknowledged — noted that this PR is superseded by #521, already merged to main.

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[bokelley]

Confirmed — #521 covers the same fix and is already on main. No further action needed here.

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Generated by Claude Code

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Generated by Claude Code

[bokelley]

Acknowledged — noted that #521 landed the same fix on main. This PR can remain closed.

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_Generated by Claude Code

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Generated by Claude Code