fix: reference module instead of tool in error message

- Change error message from 'Tool X not found' to 'Module X not found'
- More accurate since we're trying to import it as a module at this point
- Maintains existing test compatibility and error handling logic

🤖 Assisted by Amazon Q Developer

Author: mrlee-amazon

src/strands/experimental/agent_config.py

@@ -92,7 +92,7 @@ def _validate_tools(tools: list[str]) -> None:
                     )
             else:
                 raise ValueError(
-                    f"Tool '{tool}' not found. "
+                    f"Module '{tool}' not found. "
                     f"The configured tool is not annotated with @tool, and is not a module or file."
                 )