refactor: extract collection guard pattern into helper method

Extract repeated collection validation logic into a private helper method
_get_collection_or_raise() to reduce code duplication in MemoryStore.

- Created _get_collection_or_raise() helper with proper docstring
- Refactored 4 methods to use the helper:
  - _get_managed_entry
  - _put_managed_entry
  - _delete_managed_entry
  - _get_collection_keys
- Reduces 20 lines of duplicated code
- Maintains identical behavior and error messages

Addresses CodeRabbit review feedback.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: William Easton <[email protected]>

Author: github-actions[bot]

key-value/key-value-aio/src/key_value/aio/stores/memory/store.py

@@ -153,14 +153,27 @@ async def _setup_collection(self, *, collection: str) -> None:
         collection_cache = MemoryCollection(max_entries=self.max_entries_per_collection)
         self._cache[collection] = collection_cache
 
-    @override
-    async def _get_managed_entry(self, *, key: str, collection: str) -> ManagedEntry | None:
-        collection_cache: MemoryCollection | None = self._cache.get(collection)
+    def _get_collection_or_raise(self, collection: str) -> MemoryCollection:
+        """Get a collection or raise KeyError if not setup.
+
+        Args:
+            collection: The collection name.
 
+        Returns:
+            The MemoryCollection instance.
+
+        Raises:
+            KeyError: If the collection has not been setup via setup_collection().
+        """
+        collection_cache: MemoryCollection | None = self._cache.get(collection)
         if collection_cache is None:
             msg = f"Collection '{collection}' has not been setup. Call setup_collection() first."
             raise KeyError(msg)
+        return collection_cache
 
+    @override
+    async def _get_managed_entry(self, *, key: str, collection: str) -> ManagedEntry | None:
+        collection_cache = self._get_collection_or_raise(collection)
         return collection_cache.get(key=key)
 
     @override
@@ -171,32 +184,17 @@ async def _put_managed_entry(
         collection: str,
         managed_entry: ManagedEntry,
     ) -> None:
-        collection_cache: MemoryCollection | None = self._cache.get(collection)
-
-        if collection_cache is None:
-            msg = f"Collection '{collection}' has not been setup. Call setup_collection() first."
-            raise KeyError(msg)
-
+        collection_cache = self._get_collection_or_raise(collection)
         collection_cache.put(key=key, value=managed_entry)
 
     @override
     async def _delete_managed_entry(self, *, key: str, collection: str) -> bool:
-        collection_cache: MemoryCollection | None = self._cache.get(collection)
-
-        if collection_cache is None:
-            msg = f"Collection '{collection}' has not been setup. Call setup_collection() first."
-            raise KeyError(msg)
-
+        collection_cache = self._get_collection_or_raise(collection)
         return collection_cache.delete(key=key)
 
     @override
     async def _get_collection_keys(self, *, collection: str, limit: int | None = None) -> list[str]:
-        collection_cache: MemoryCollection | None = self._cache.get(collection)
-
-        if collection_cache is None:
-            msg = f"Collection '{collection}' has not been setup. Call setup_collection() first."
-            raise KeyError(msg)
-
+        collection_cache = self._get_collection_or_raise(collection)
         return collection_cache.keys(limit=limit)
 
     @override