fix(examples): fix 6 storyboard compliance gaps in seller_agent.py

[bokelley]

Refs #304

Fixes items 1–6 from the issue punch list. Item 7 (seed_product controller stub) is deferred pending #296, which adds the seed_* dispatcher to TestControllerStore. The controller_detected: true target also depends on #282 landing.

What changed in examples/seller_agent.py:

1. Idempotency capability — adcp.idempotency is required by the capabilities schema; added idempotency={"supported": False} to capabilities_response(). Without it the runner silently downgrades to v2 mode.
2. get_media_buys missing total_budget — schema required: ["total_budget"]; added sum of package budgets (or 0 for flat-rate packages).
3. create_media_buy status lifecycle — return pending_creatives when no per-package creatives or creative_assignments are supplied at create time; active otherwise. Checks both creative-attachment paths on the input request packages.
4. list_creative_formats renders schema — role is required; width/height must be nested under dimensions. Changed bare {"width": W, "height": H} to {"role": "primary", "dimensions": {"width": W, "height": H}}.
5. list_creative_formats filter ignored — honor format_ids request parameter; filter on compound (agent_url, id) key per format-id.json schema; absent means return all.
6. update_media_buy missing PACKAGE_NOT_FOUND — validate package_id existence when a packages update is requested; return PACKAGE_NOT_FOUND for unknown IDs (per error-code.json canonical code). Note: this reference implementation validates but does not apply budget/targeting writes back — a production seller should merge update fields into the stored package.

What was tested

• pytest tests/ -q --ignore=tests/conformance/signing/test_ip_pinned_transport.py --ignore=tests/integration — 2132 passed, 0 failures (no tests touch examples/, suite clean)
• python -c "import ast; ast.parse(open('examples/seller_agent.py').read())" — syntax OK
• examples/ is excluded from ruff and mypy per pyproject.toml

Pre-PR review

• code-reviewer: approved — no CI-blocking issues; two fixes applied from review (static error message in PACKAGE_NOT_FOUND to avoid user-data in LLM-visible error context per adcp_error docstring; .get() in format_ids filter to avoid KeyError on malformed input)
• ad-tech-protocol-expert: approved — all 6 items schema-sound; blocker fixed (creative_assignments path now covered alongside creatives in has_creatives check; confirmed exhaustive per package-request.json schema)

Nits surfaced (not fixed — follow-up candidates)

• update_media_buy silently accepts valid packages update payloads but discards the field writes; a production seller should merge pkg_update fields back into stored packages (comment added in-code)
• total_budget: 0.0 is schema-valid (minimum: 0) for flat-rate-only catalogs but may surprise implementors — inline comment added

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Session: https://claude.ai/code/session_01GDFGtecoKD34z3KmZ4GuML

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[bokelley]

Superseded by #310 — same 6-item fix but with cleaner field-order priority (creative_assignments or creatives per AdCP v3) and valid_actions populated on pending_creatives responses (caught in triage re-review).

[bokelley]

Acknowledged — #310 supersedes this PR with the creative_assignments or creatives field-order fix and valid_actions on pending_creatives responses. No further action needed here.

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