fix(decisioning): handle Specialism|str union in create_media_buy_store

Post-rebase fix. ``create_media_buy_store`` (added in adcontextprotocol#474, landed
after this branch was originally cut) intersects
``capabilities.specialisms`` against a string set to gate
``targeting_overlay`` echo on the seller's ``property-lists`` /
``collection-lists`` claim.

Per adcontextprotocol#479, ``capabilities.specialisms`` is now ``list[Specialism | str]``
— spec-known slugs are coerced to ``Specialism`` enum members at
construction. The naive ``set(capabilities.specialisms) &
_OVERLAY_ECHO_SPECIALISMS`` was an empty intersection because
``Specialism.property_lists != "property-lists"``, silently routing
every adopter to the no-op wrapper regardless of their declaration.

Fix: extract slug uniformly via ``.value`` (with the same hasattr
pattern other readers use) before intersecting. The wrapper now
correctly activates when the seller declares either spec slug,
whether passed as enum or string.

10 ``test_media_buy_store.py`` tests pass; 3,469 unit tests overall.

Refs adcontextprotocol#479

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

Author: bokelley

src/adcp/decisioning/media_buy_store.py

@@ -188,7 +188,14 @@ def create_media_buy_store(
     assignment site (``platform.media_buy_store = ...``) without
     aliasing the underlying persistence layer.
     """
-    claimed = set(capabilities.specialisms) & _OVERLAY_ECHO_SPECIALISMS
+    # ``capabilities.specialisms`` is ``list[Specialism | str]`` per #479
+    # — spec-known slugs are coerced to ``Specialism`` enum members at
+    # construction; novel/typo slugs pass through as strings. Extract the
+    # slug uniformly before set intersection.
+    declared_slugs = {
+        entry.value if hasattr(entry, "value") else entry for entry in capabilities.specialisms
+    }
+    claimed = declared_slugs & _OVERLAY_ECHO_SPECIALISMS
     if claimed:
         return _ActiveMediaBuyStore(adopter_store)
     return _NoopMediaBuyStore(adopter_store)