fixup(hello_seller): list_creatives spec-valid envelope + model_validate tests

Per review on #531:
- list_creatives stub returned {"creatives": []} which is missing the
  spec-required query_summary and pagination fields. A buyer hitting
  the running example would get a non-conformant ListCreativesResponse.
  Now returns a minimal valid envelope with an explanatory docstring
  pointing adopters at the post-filter count and cursor state.
- Smoke tests now run ``Response.model_validate(resp)`` against the
  canonical Pydantic model so spec drift fails the test, not just
  dict-key presence.
- Class docstring lede: "all nine required methods" → "all nine
  methods" (four are soft-required, distinction is two lines below).

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

Author: bokelley

examples/hello_seller.py

@@ -85,7 +85,7 @@
 class HelloSeller(DecisioningPlatform):
     """The canonical v6.0 sales-non-guaranteed adopter.
 
-    Implements all nine required methods of ``sales-non-guaranteed``: the
+    Implements all nine methods of the ``sales-non-guaranteed`` surface: the
     five hard-required (``get_products``, ``create_media_buy``,
     ``update_media_buy``, ``sync_creatives``, ``get_media_buy_delivery``)
     and the four soft-required by the SalesPlatform Protocol in v6.0 rc.1+
@@ -288,8 +288,17 @@ def list_creatives(
         Return all creatives the buyer has synced. Wire to your creative
         asset store in production; buyers use this to check approval
         statuses and discover available creatives.
+
+        ``query_summary`` and ``pagination`` are required by the spec
+        envelope — fill ``total_matching``/``returned`` from the
+        post-filter result count and ``has_more`` from the cursor state
+        once you have a real store.
         """
-        return {"creatives": []}
+        return {
+            "query_summary": {"total_matching": 0, "returned": 0},
+            "pagination": {"has_more": False},
+            "creatives": [],
+        }
 
     def provide_performance_feedback(
         self,

tests/test_hello_seller_integration.py

@@ -259,35 +259,42 @@ async def test_advertised_tools_class_attribute_set(
 
 
 @pytest.mark.asyncio
-async def test_get_media_buys_returns_empty_list(handler: PlatformHandler) -> None:
-    """Smoke: stub returns valid wire shape for get_media_buys."""
-    from adcp.types import GetMediaBuysRequest
+async def test_get_media_buys_returns_spec_valid_envelope(handler: PlatformHandler) -> None:
+    """Stub returns a wire shape that satisfies ``GetMediaBuysResponse``."""
+    from adcp.types import GetMediaBuysRequest, GetMediaBuysResponse
 
     req = GetMediaBuysRequest(account={"account_id": "buyer-1"})
     resp = await handler.get_media_buys(req, ToolContext())
-    assert isinstance(resp, dict)
+    # Validate against the canonical Pydantic model — catches drift
+    # between stub and spec, not just dict-key presence.
+    GetMediaBuysResponse.model_validate(resp)
     assert resp["media_buys"] == []
 
 
 @pytest.mark.asyncio
-async def test_list_creative_formats_returns_empty_list(handler: PlatformHandler) -> None:
-    """Smoke: stub returns valid wire shape for list_creative_formats."""
-    from adcp.types import ListCreativeFormatsRequest
+async def test_list_creative_formats_returns_spec_valid_envelope(
+    handler: PlatformHandler,
+) -> None:
+    """Stub returns a wire shape that satisfies ``ListCreativeFormatsResponse``."""
+    from adcp.types import ListCreativeFormatsRequest, ListCreativeFormatsResponse
 
     req = ListCreativeFormatsRequest()
     resp = await handler.list_creative_formats(req, ToolContext())
-    assert isinstance(resp, dict)
+    ListCreativeFormatsResponse.model_validate(resp)
     assert resp["formats"] == []
 
 
 @pytest.mark.asyncio
-async def test_list_creatives_returns_empty_list(handler: PlatformHandler) -> None:
-    """Smoke: stub returns valid wire shape for list_creatives."""
-    from adcp.types import ListCreativesRequest
+async def test_list_creatives_returns_spec_valid_envelope(handler: PlatformHandler) -> None:
+    """Stub returns a wire shape that satisfies ``ListCreativesResponse``,
+    including the spec-required ``query_summary`` and ``pagination``
+    envelopes (a buyer hitting the example otherwise gets a non-conformant
+    response)."""
+    from adcp.types import ListCreativesRequest, ListCreativesResponse
 
     req = ListCreativesRequest(account={"account_id": "buyer-1"})
     resp = await handler.list_creatives(req, ToolContext())
-    assert isinstance(resp, dict)
+    ListCreativesResponse.model_validate(resp)
     assert resp["creatives"] == []