fix(examples/multi_platform_seller): align mocks with wire contract; storyboard gate now blocking

The two mock platforms behind PlatformRouter (MockGuaranteedPlatform,
MockNonGuaranteedPlatform) were silently failing the AdCP storyboard
suite since PR #490 — masked by ``continue-on-error: true`` on the CI
job. Pre-fix counts: tenant-a 7 passed / 8 failed / 44 skipped (cascade
from create failures), tenant-b 12 passed / 6 failed / 41 skipped.

Wire-contract alignments:

* ``create_media_buy``: read ``packages[].product_id`` (singular,
  per ``schemas/3.0.6/media-buy/package-request.json``) instead of
  the non-existent ``packages[].products`` array. ``budget`` is a
  flat number, not a nested ``{total: ...}``.
* ``sync_creatives``: response items use ``{creative_id, action:
  "created", status: "approved"}`` per
  ``schemas/3.0.6/creative/sync-creatives-response.json``. The old
  ``approval_status`` key wasn't a valid field.
* ``get_media_buys``: implemented on both mocks (was missing,
  causing tenant-b's "'media_buys' is a required property"). Echoes
  ``targeting_overlay`` persisted at create / update time so the
  ``inventory_list_targeting`` storyboard can verify round-trip.
* ``update_media_buy``: validates ``packages[].package_id`` against
  the buy's persisted packages and raises ``PACKAGE_NOT_FOUND``
  (recovery=correctable) on unknown ids; persists ``targeting_overlay``
  + ``measurement_terms`` patches for round-trip.
* Both mocks now check buyer-proposed ``measurement_terms`` and raise
  ``TERMS_REJECTED`` when ``max_variance_percent < 5`` or
  ``measurement_window`` is outside ``c3``/``c7`` — mirrors the v3
  reference seller's policy in ``examples/seller_agent.py``.
* MockNonGuaranteedPlatform now starts buys in ``pending_creatives``
  and advances to ``active`` via ``pending_start`` only after
  creatives sync, so the ``pending_creatives_to_start`` storyboard
  passes. The state machine doesn't permit a direct
  pending_creatives → active jump.
* ``MEDIA_BUY_NOT_FOUND`` recovery changed from ``terminal`` to
  ``correctable`` to match spec semantics — the buyer can retry with
  a fresh id.

Storyboards skipped by capability gate (correct behavior, no change):
``proposal_finalize`` is gated by ``media_buy.supports_proposals``.
Neither mock declares it because neither implements the proposal
lifecycle. The gate stays as-is until ProposalManager v1.5 ships.

CI: drops ``continue-on-error: true`` from the
``storyboard-multi-platform-seller`` job. Future drift in the mocks
or the spec now blocks merge. Artifact upload still runs on
``if: always()`` so failures remain debuggable.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bokelley

.github/workflows/ci.yml

@@ -652,10 +652,11 @@ jobs:
     runs-on: ubuntu-latest
     # Multi-tenant proof: one process, two tenants, one router. Each
     # tenant's storyboard runs against its own subdomain
-    # (tenant-a.localhost / tenant-b.localhost). Non-blocking until
-    # the storyboard reaches passing — same posture as the
-    # examples/seller_agent.py job above.
-    continue-on-error: true
+    # (tenant-a.localhost / tenant-b.localhost). Blocking gate — both
+    # tenants must pass the AdCP storyboard suite for the example to
+    # remain a credible reference. continue-on-error was dropped after
+    # the mocks were aligned with the wire contract; future drift now
+    # blocks CI.
 
     steps:
       - uses: actions/checkout@v4