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/image_rag.py

@@ -169,7 +169,7 @@ class ImageRAG(BaseRAGExample):
         print(f"✅ Processed {len(image_data)} images")
         return image_data
 
-    async def build_index(self, args, texts: list[str]) -> str:
+    async def build_index(self, args, texts: list[dict[str, Any]]) -> str:
         """Build index using pre-computed CLIP embeddings."""
         from leann.api import LeannBuilder