feat(storage): add get_value method to MongoStorage

Implement get_value method for MongoStorage that retrieves
individual values from storage data using MongoDB projections.

Add comprehensive test coverage for the new method.

tests/test_fsm/storage/test_mongo.py

@@ -149,6 +149,88 @@ class TestStateAndDataDoNotAffectEachOther:
         }
 
 
+class TestGetValue:
+    async def test_get_existing_value(
+        self,
+        mongo_storage: MongoStorage,
+        storage_key: StorageKey,
+    ):
+        await mongo_storage.set_data(
+            storage_key, {"key": "value", "number": 42, "list": [1, 2, 3]}
+        )
+
+        assert await mongo_storage.get_value(storage_key, "key") == "value"
+        assert await mongo_storage.get_value(storage_key, "number") == 42
+        assert await mongo_storage.get_value(storage_key, "list") == [1, 2, 3]
+
+    async def test_get_non_existing_value(
+        self,
+        mongo_storage: MongoStorage,
+        storage_key: StorageKey,
+    ):
+        await mongo_storage.set_data(storage_key, {"key": "value"})
+
+        assert await mongo_storage.get_value(storage_key, "non_existing_key") is None
+        assert (
+            await mongo_storage.get_value(storage_key, "non_existing_key", default="default")
+            == "default"
+        )
+
+    async def test_get_value_from_non_existing_document(
+        self,
+        mongo_storage: MongoStorage,
+        storage_key: StorageKey,
+    ):
+        assert await mongo_storage._collection.find_one({}) is None
+
+        assert await mongo_storage.get_value(storage_key, "any_key") is None
+        assert (
+            await mongo_storage.get_value(storage_key, "any_key", default="default") == "default"
+        )
+
+    async def test_get_value_with_document_containing_only_state(
+        self,
+        mongo_storage: MongoStorage,
+        storage_key: StorageKey,
+    ):
+        await mongo_storage.set_state(storage_key, "test")
+
+        document = await mongo_storage._collection.find_one({})
+        assert document is not None
+        assert "data" not in document
+
+        assert await mongo_storage.get_value(storage_key, "any_key") is None
+        assert (
+            await mongo_storage.get_value(storage_key, "any_key", default="default") == "default"
+        )
+
+    async def test_get_value_uses_projection(
+        self,
+        mongo_storage: MongoStorage,
+        storage_key: StorageKey,
+        monkeypatch,
+    ):
+        await mongo_storage.set_data(
+            storage_key, {"key1": "value1", "key2": "value2", "key3": {"nested": "data"}}
+        )
+
+        original_find_one = mongo_storage._collection.find_one
+        calls = []
+
+        async def mock_find_one(*args, **kwargs):
+            calls.append(kwargs)
+            return await original_find_one(*args, **kwargs)
+
+        monkeypatch.setattr(mongo_storage._collection, "find_one", mock_find_one)
+
+        value = await mongo_storage.get_value(storage_key, "key2")
+
+        assert len(calls) == 1
+        assert "projection" in calls[0]
+        assert calls[0]["projection"] == {"_id": 0, "data.key2": 1}
+        assert value == "value2"
+
+
 @pytest.mark.parametrize(
     "value,result",
     [