Handle truncation when single long non-system chat message

Previously was assuming the system prompt is being always passed as
the first message. So expected there to be at least 2 messages in logs.

This broke chat actors querying with single long non system message.

A more robust way to extract system prompt is via the message role
instead

src/khoj/processor/conversation/utils.py

@@ -199,19 +199,26 @@ def truncate_messages(
             f"Fallback to default chat model tokenizer: {default_tokenizer}.\nConfigure tokenizer for unsupported model: {model_name} in Khoj settings to improve context stuffing."
         )
 
-    system_message = messages.pop()
-    assert type(system_message.content) == str
-    system_message_tokens = len(encoder.encode(system_message.content))
+    # Extract system message from messages
+    system_message = None
+    for idx, message in enumerate(messages):
+        if message.role == "system":
+            system_message = messages.pop(idx)
+            break
+
+    system_message_tokens = (
+        len(encoder.encode(system_message.content)) if system_message and type(system_message.content) == str else 0
+    )
 
     tokens = sum([len(encoder.encode(message.content)) for message in messages if type(message.content) == str])
+
+    # Drop older messages until under max supported prompt size by model
     while (tokens + system_message_tokens) > max_prompt_size and len(messages) > 1:
         messages.pop()
-        assert type(system_message.content) == str
         tokens = sum([len(encoder.encode(message.content)) for message in messages if type(message.content) == str])
 
     # Truncate current message if still over max supported prompt size by model
     if (tokens + system_message_tokens) > max_prompt_size:
-        assert type(system_message.content) == str
         current_message = "\n".join(messages[0].content.split("\n")[:-1]) if type(messages[0].content) == str else ""
         original_question = "\n".join(messages[0].content.split("\n")[-1:]) if type(messages[0].content) == str else ""
         original_question = f"\n{original_question}"
@@ -223,7 +230,7 @@ def truncate_messages(
         )
         messages = [ChatMessage(content=truncated_message + original_question, role=messages[0].role)]
 
-    return messages + [system_message]
+    return messages + [system_message] if system_message else messages
 
 
 def reciprocal_conversation_to_chatml(message_pair):