fix(examples): seller_agent.py AdCP 3.0.1 storyboard compliance (items 1-6 of #304)

[bokelley]

Refs #304

Fixes 6 content-side storyboard failures in examples/seller_agent.py (the SDK's reference seller for the media_buy_seller compliance suite). All changes are examples-only — no SDK source modifications. Item 7 (seed_product / controller_detected) remains blocked on #282.

Changes

1. adcp.idempotency not declared — add idempotency={"supported": False} to capabilities_response(). Without this, the AdCP 3.0.1 storyboard runner downgrades to v2 mode.

2. get_media_buys missing total_budget — schema requires this field; added as sum(pkg budgets) per media buy entry.

3. create_media_buy returns active without creatives — now returns pending_creatives when no creative_assignments/creatives are in the request packages. update_media_buy transitions pending_creatives → active when creatives are attached. Also passes explicit valid_actions for pending_creatives status because MEDIA_BUY_STATE_MACHINE on main does not yet include this key (it lands with fix(server): fix streamable-http ASGI error, host binding, and AdCP 3.0.1 scenario gaps #296).

4. list_creative_formats renders missing role — {"width":300,"height":250} → {"role":"primary","dimensions":{"width":300,"height":250}} per schema.

5. list_creative_formats filter ignored — now honours format_ids filter from request, matching on compound (agent_url, id) key.

6. update_media_buy with bogus package_id — validates package IDs exist before processing; returns PACKAGE_NOT_FOUND if not.

Also fixed during expert review:

• DemoStore.simulate_delivery had reported_spend: float | None instead of dict[str, Any] | None, mismatching the base class and the ReportedSpend schema.

What was tested

• pytest tests/ -q --ignore=tests/conformance/signing/test_ip_pinned_transport.py — 2223 passed, 22 skipped (run before dev-dep install; examples/ has no unit tests, so all SDK tests remain unaffected)
• ruff check src/ — clean (examples/ excluded per pyproject.toml)
• python3 -c "import ast; ast.parse(...)" — syntax OK on final diff

Pre-PR review

• code-reviewer: approved — confirmed no blockers after valid_actions fix; nit: silent empty-list for cross-agent agent_url mismatches in format filter (noted below)
• dx-expert: approved — all 6 compliance fixes correct; flagged pending_creatives valid_actions gap (fixed in commit 3) and simulate_delivery type mismatch (fixed in commit 2)

Nits surfaced (not fixed — follow-up candidates):

• list_creative_formats filter silently returns empty list when agent_url doesn't match this seller's URL; a comment explaining the match semantics would help readers
• get_media_buy_delivery returns hardcoded numbers that don't reflect simulate_delivery state (pre-existing gap, out of scope for examples/seller_agent.py: storyboard content gaps exposed by #296 transport fix #304)
• inject_context is never called in handlers (pre-existing)

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_01HAP5upax2a7FrcrmgVwTX2

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