Make organic web search result text snippet field optional

All web search providers, like Jina/Searxng?, do not return a text
snippet. Making snippet optional allows processing search results by
these web search providers, without hitting validation errors.

src/khoj/database/models/__init__.py

@@ -75,7 +75,7 @@ class KnowledgeGraph(PydanticBaseModel):
 
 
 class OrganicContext(PydanticBaseModel):
-    snippet: str
+    snippet: Optional[str] = None
     title: str
     link: str