Make URI field optional for now to handle previously saved documents

For files not synced after the previous release, context uri is unset.
This results in failure to save chat messages that retrieve documents
as the uri field cannot be unset so pre save validation fails.

We'd use a db migration to handle this but this is a quick mitigation
for now.

src/khoj/database/models/__init__.py

@@ -23,7 +23,7 @@ logger = logging.getLogger(__name__)
 class Context(PydanticBaseModel):
     compiled: str
     file: str
-    uri: str
+    uri: Optional[str] = None
     query: Optional[str] = None
 
     @model_validator(mode="after")

src/khoj/processor/conversation/utils.py

@@ -646,7 +646,11 @@ def generate_chatml_messages_with_context(
 
         if not is_none_or_empty(chat.context):
             references = "\n\n".join(
-                {f"# URI: {item.uri}\n## {item.compiled}\n" for item in chat.context or [] if isinstance(item, dict)}
+                {
+                    f"# URI: {item.uri or item.file}\n## {item.compiled}\n"
+                    for item in chat.context or []
+                    if isinstance(item, dict)
+                }
             )
             message_context += [{"type": "text", "text": f"{prompts.notes_conversation.format(references=references)}"}]