feat(examples): DemoStore overrides for force_create_media_buy_arm, force_task_completion, and seed_* scenarios

[bokelley]

Closes #312

DemoStore in examples/seller_agent.py now overrides all 7 new TestControllerStore methods landed in #282 (force_create_media_buy_arm, force_task_completion) and #296 (seed_product, seed_pricing_option, seed_creative, seed_plan, seed_media_buy). Before this PR, _list_scenarios could not detect any of these methods, so the storyboard runner reported controller_detected: false and 5 fixture-dependent storyboard steps failed.

What changed

Module-level state (examples/seller_agent.py)

• _DEFAULT_ACCOUNT_ID = "__default__" — shared sentinel for the single-tenant demo; documented with a note that real sellers must scope by account_id.
• plans dict — backing store for seed_plan.
• pending_directives — single-shot arm directives, keyed by account_id, consumed by the next create_media_buy call from that account.
• pending_task_completions — task registry keyed by task_id; entries created when create_media_buy processes a submitted directive.

DemoSeller.get_adcp_capabilities — compliance_testing.scenarios list extended with the 7 new scenario names.

DemoSeller.create_media_buy — checks pending_directives at entry; pops the directive (single-shot), then:

• arm='submitted' → registers the task in pending_task_completions and returns the submitted-task envelope {"status": "submitted", "task_id": ..., "message"?: ...}.
• arm='input-required' → returns {"reason": "APPROVAL_REQUIRED"} (CreateMediaBuyInputRequired per AdCP spec).
• No directive → falls through to the existing sync-success path.

DemoStore — new overrides:

• force_create_media_buy_arm: stores the directive; forced.task_id included only for arm='submitted'.
• force_task_completion: resolves task to completed; raises NOT_FOUND for unknown task_ids or cross-account callers; idempotent on identical-params replay (echoes original previous_state); raises INVALID_TRANSITION on diverging-params replay against an already-completed task.
• seed_product: append or replace in PRODUCTS list; create_media_buy's valid_ids set is computed fresh per call so new products are immediately available.
• seed_pricing_option: attach to specified product (or first product when product_id=None); raises NOT_FOUND when the specified product doesn't exist.
• seed_creative: upsert into creatives dict.
• seed_plan: upsert into plans dict.
• seed_media_buy: upsert into media_buys dict; stamps media_buy_id into the stored dict (consistent with seed_creative / seed_plan).

What was tested

• pytest tests/ -q -m "not integration" --ignore=tests/conformance/signing/test_ip_pinned_transport.py: 2321 passed, 22 skipped, 1 xfailed — no regressions. (The ignored test hits example.com over the network and fails with 403 in this sandbox; it's pre-existing and unrelated to this diff.)
• examples/ is excluded from ruff, mypy, and pytest in pyproject.toml — no linting or type-check applies to the example file itself.

Pre-PR review

• code-reviewer: approved after two blockers fixed — (1) force_task_completion now detects diverging-params replay on completed tasks and raises INVALID_TRANSITION; (2) seed_media_buy now stamps media_buy_id into the stored dict (consistent with all other seed_* methods). 1 nit noted: seed_pricing_option error message when product_id=None and PRODUCTS is empty reads Product 'None' not found.
• dx-expert: approved after three DX issues addressed — _DEFAULT_ACCOUNT_ID extracted to module-level constant (was duplicated 3×); arm-dispatch branches in create_media_buy annotated with spec shape; idempotent-replay previous_state now echoes the original value from first completion. Remaining nits surfaced below.

Nits surfaced (not fixed — follow-up candidates)

• seed_pricing_option reports Product 'None' not found when product_id=None and PRODUCTS is empty. Low-severity; the storyboard always passes a product_id.
• seed_pricing_option silently attaches to the first product when product_id=None; a comment noting this behavior would help copiers.
• Capabilities scenario list order diverges slightly from SCENARIOS in test_controller.py; force_session_status absence not annotated.
• context kwarg on new DemoStore methods is accepted but unused; a comment would clarify it's an optional SDK hook.

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_01DJWM1a9nfjauGxSks9T1KW

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