fix: Properly handle prompt=None & avoid agent hanging

bedrock.py now catches all exceptions in _stream so it no longer hangs when invalid content is passed.

In addition, since we don't allow agent(None), go ahead and validate that none is not passed throughout our agent calls.

Author: zastrowm

src/strands/agent/agent.py

@@ -458,7 +458,7 @@ async def structured_output_async(
 
             # Create temporary messages array if prompt is provided
             if prompt:
-                temp_messages = self.messages + self._standardize_prompt(prompt)
+                temp_messages = self.messages + [self._standardize_prompt(prompt)]
             else:
                 temp_messages = self.messages