fix: incorrect MessageResult type on forgotton message flow

deployment/docker-build/dev/Dockerfile

@@ -52,7 +52,8 @@ COPY deployment/migrations ./deployment/migrations
 COPY deployment/scripts ./deployment/scripts
 COPY .git ./.git
 COPY src ./src
-RUN pip install -e .
+
+RUN pip install -e .[linting]
 RUN pip install hatch
 
 FROM base

src/aleph/handlers/message_handler.py

@@ -39,8 +39,9 @@
 from aleph.toolkit.timestamp import timestamp_to_datetime
 from aleph.types.db_session import DbSession, DbSessionFactory
 from aleph.types.files import FileType
-from aleph.types.message_processing_result import ProcessedMessage
+from aleph.types.message_processing_result import ProcessedMessage, RejectedMessage
 from aleph.types.message_status import (
+    ErrorCode,
     InvalidMessageException,
     InvalidMessageFormat,
     InvalidSignature,
@@ -378,7 +379,7 @@ async def verify_and_fetch(
 
     async def process(
         self, session: DbSession, pending_message: PendingMessageDb
-    ) -> ProcessedMessage:
+    ) -> ProcessedMessage | RejectedMessage:
         """
         Process a pending message.
 
@@ -415,7 +416,10 @@ async def process(
                 forgotten_message=forgotten_message,
                 pending_message=pending_message,
             )
-            return ProcessedMessage(message=forgotten_message, is_confirmation=True)
+            return RejectedMessage(
+                pending_message=pending_message,
+                error_code=ErrorCode.FORGET_TARGET_NOT_FOUND,
+            )
 
         message = await self.verify_and_fetch(
             session=session, pending_message=pending_message

tests/message_processing/test_process_forgotten_messages.py

@@ -9,6 +9,8 @@
 from aleph.handlers.message_handler import MessageHandler
 from aleph.storage import StorageService
 from aleph.types.db_session import DbSessionFactory
+from aleph.types.message_processing_result import ProcessedMessage, RejectedMessage
+from aleph.types.message_status import ErrorCode
 
 from .load_fixtures import load_fixture_message_list
 
@@ -47,7 +49,7 @@ async def test_duplicated_forgotten_message(
             session=session,
             pending_message=m1,
         )
-        assert test1
+        assert isinstance(test1, ProcessedMessage)
 
         res1 = cast(
             MessageDb,
@@ -60,7 +62,7 @@ async def test_duplicated_forgotten_message(
             session=session,
             pending_message=m2,
         )
-        assert test2
+        assert isinstance(test2, ProcessedMessage)
 
         res2 = cast(
             MessageDb,
@@ -73,7 +75,8 @@ async def test_duplicated_forgotten_message(
             session=session,
             pending_message=m3,
         )
-        assert test3
+        assert isinstance(test3, RejectedMessage)
+        assert test3.error_code == ErrorCode.FORGET_TARGET_NOT_FOUND
 
         res3 = cast(
             MessageDb,