Fix remaining ty type errors

- Fix slack_mcp_reader.py channel parameter can be None
- Fix embedding_compute.py ContextProp type issue
- Fix searcher_base.py method override signatures
- Fix chunking_utils.py chunk_text assignment
- Fix slack_rag.py and twitter_rag.py return types
- Fix email.py and image_rag.py method overrides

apps/slack_rag.py

@@ -189,7 +189,8 @@ class SlackMCPRAG(BaseRAGExample):
                 print(sample_text)
                 print("-" * 40)
 
-            return texts
+            # Convert strings to dict format expected by base class
+            return [{"text": text, "metadata": {"source": "slack"}} for text in texts]
 
         except Exception as e:
             print(f"Error loading Slack data: {e}")