feat: harden shared platform contracts

tests/test_postbox_contract.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import unittest
+from datetime import datetime, timezone
 
 from govoplan_core.core.modules import ModuleContext, ModuleManifest
 from govoplan_core.core.postbox import (
@@ -11,6 +12,7 @@ from govoplan_core.core.postbox import (
     CAPABILITY_POSTBOX_MESSAGES,
     PostboxAccessProvider,
     PostboxDeliveryProvider,
+    PostboxDeliveryReceiptSummaryRef,
     PostboxDirectoryProvider,
     PostboxEvidenceProvider,
     PostboxMessagesProvider,
@@ -69,6 +71,17 @@ class _PostboxProvider:
         del session, tenant_id, message_id, attachment
         return None
 
+    def delivery_receipt_summaries(
+        self,
+        session,
+        *,
+        tenant_id,
+        producer_module,
+        delivery_ids,
+    ):
+        del session, tenant_id, producer_module, delivery_ids
+        return {}
+
 
 class PostboxContractTests(unittest.TestCase):
     def test_protocols_and_registry_helpers_resolve_without_module_imports(self) -> None:
@@ -78,6 +91,16 @@ class PostboxContractTests(unittest.TestCase):
         self.assertIsInstance(provider, PostboxMessagesProvider)
         self.assertIsInstance(provider, PostboxDeliveryProvider)
         self.assertIsInstance(provider, PostboxEvidenceProvider)
+        self.assertEqual(
+            "delivery-1",
+            PostboxDeliveryReceiptSummaryRef(
+                delivery_id="delivery-1",
+                message_id="message-1",
+                postbox_id="postbox-1",
+                delivery_status="accepted",
+                accepted_at=datetime.now(timezone.utc),
+            ).delivery_id,
+        )
 
         registry = PlatformRegistry()
         registry.register(