feat(examples): support @adcp/sdk 7.10 storyboards

[sangilish]

Summary

Refs #782.

This moves the example storyboard jobs to @adcp/sdk@7.10.1 and catches the multi-platform seller example up to the stricter 7.10 storyboard checks.

What changed

• Added a shared ADCP_SDK_VERSION: "7.10.1" workflow pin and versioned npm cache keys for all storyboard jobs.
• Added list_accounts support to the multi-platform demo account store so sales specialisms advertise an account-discovery tool.
• Updated both multi-platform mock sellers to emit storyboard-compatible UTC timestamps, preserve structured creative format_id, honor list_creatives creative-id filters, and return NOT_CANCELLABLE for repeated cancellation.
• Added regression coverage for account-discovery advertising, media-buy timestamp projection, repeated cancel behavior, and creative format/filter projection.
• Documented the example catch-up in the changelog.

Testing

• uv run pytest tests/ -q — 4943 passed, 34 skipped, 9 deselected, 1 xfailed
• uv run ruff check examples/multi_platform_seller/src/account_store.py examples/multi_platform_seller/src/mock_guaranteed.py examples/multi_platform_seller/src/mock_non_guaranteed.py tests/test_multi_platform_seller_example.py
• uv run mypy src/adcp/
• adcp storyboard run http://127.0.0.1:3001/mcp media_buy_seller --json --allow-http with @adcp/sdk@7.10.1 — passing, 57 passed / 0 failed
• adcp storyboard run http://tenant-a.localhost:3001/mcp media_buy_seller --json --allow-http with @adcp/sdk@7.10.1 — passing, 31 passed / 0 failed
• adcp storyboard run http://tenant-b.localhost:3001/mcp media_buy_seller --json --allow-http with @adcp/sdk@7.10.1 — passing, 31 passed / 0 failed
• adcp storyboard run http://127.0.0.1:3003/mcp media_buy_seller/proposal_finalize --json --allow-http with @adcp/sdk@7.10.1 — passing, 5 passed / 0 failed
• adcp storyboard run http://acme.localhost:3001/mcp media_buy_seller --auth dev-bearer-token-acme-1 --json --allow-http with @adcp/sdk@7.10.1 and local Postgres/mock-server — passing, 32 passed / 0 failed; upstream traffic total 44

Draft until GitHub Actions confirms the full matrix.