feat(test-controller): add seed_creative_format scenario; advertise force_session_status in capabilities

[bokelley]

Refs #314

Summary

Investigation of #314 found two Python SDK gaps that together explain the controller_detected: false problem (or are latent bugs that will surface imminently):

1. seed_creative_format was missing from the Python SDK entirely. The comply-test-controller-request.json schema (AdCP 3.0.1) includes this scenario but it was absent from SCENARIOS, TestControllerStore, the dispatcher, and DemoStore. Any storyboard that seeds a creative format would receive UNKNOWN_SCENARIO.

2. force_session_status was not advertised in DemoSeller's static compliance_testing.scenarios field. The get-adcp-capabilities-response.json schema allows exactly 6 scenario values in the static field; DemoSeller was only advertising 5 (missing force_session_status). The JS storyboard runner likely gates controller_detected on a check of this static field — it would not see the full set and flip the flag. Adding force_session_status to the static list and providing a minimal DemoStore stub brings the static and dynamic (list_scenarios) surfaces into alignment for this scenario.

What this PR does not fix: The newer scenarios (force_create_media_buy_arm, force_task_completion, seed_*) cannot be advertised in the static capabilities field until the get-adcp-capabilities-response.json schema enum is extended. That is a cross-repo fix against the AdCP spec; a follow-up issue should be filed against adcontextprotocol/adcp requesting the enum update.

Changes

• src/adcp/server/test_controller.py: adds "seed_creative_format" to SCENARIOS, adds TestControllerStore.seed_creative_format() base method, adds dispatcher branch
• examples/seller_agent.py: adds "force_session_status" to DemoSeller static capabilities, adds DemoStore.force_session_status() stub, adds DemoStore.seed_creative_format() implementation backed by seeded_creative_formats module dict, merges seeded formats into list_creative_formats response
• tests/test_server_dx.py: three new tests — test_seed_creative_format_dispatches, test_seed_creative_format_in_list_scenarios, test_seed_creative_format_structured_fixture_id

What was tested

• pytest tests/test_server_dx.py tests/test_test_controller_context.py -v → 58 passed, 0 failed
• ruff check src/adcp/server/test_controller.py → clean
• mypy src/adcp/server/test_controller.py → no issues

Pre-PR review

Pre-PR review:

• code-reviewer: approved — found 1 blocker (fixed before PR open: data.get("format_id") could return a dict from a structured fixture, making it an unhashable dict key; fixed to (data.get("format_id") or {}).get("id")). Also noted test coverage gap for the structured-fixture path (added as third test in final commit).
• ad-tech-protocol-expert: approved — force_session_status is schema-legal for a media_buy-only seller (in the 6-value capabilities enum). seed_creative_format correctly mirrors the pattern of other seed_* methods. Confirmed the blocker (dict-as-key) was already fixed. No additional blockers.

Nits (not fixed, noted for reviewer):

• DemoStore.force_session_status always returns previous_state: "active". Acceptable for a demo seller that has no SI session state; multi-step storyboards that issue two force_session_status calls in sequence will see a misleading value on the second call. Production stores should track actual session state.
• seeded_creative_formats is module-level state that is never cleared between compliance sessions (consistent with how all other module-level dicts in seller_agent.py behave, but worth noting since it influences what the protocol advertises as available formats).
• list_creative_formats concatenates without deduplication. A seeded format_id colliding with a static format (display_300x250, display_970x250) would appear twice in the unfiltered response. Unlikely in practice given storyboard seed IDs.
• TestControllerStore.seed_creative_format docstring notes the seller MUST wire the seeded ID into list_creative_formats but doesn't call this out as a subclass responsibility — future implementors could override without wiring the list method.

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> →
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  issue.

See adcp#3121
for context.

Session: https://claude.ai/code/session_018i5iWUxUayQEh5K3v9HZRG