Fix Python ty type error in session transform handler

Use str() to ensure transform callback result is typed as str,
fixing the invalid-assignment error from ty type checker at
session.py:689.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

python/copilot/session.py

@@ -686,7 +686,7 @@ async def _handle_system_message_transform(
                     transformed = callback(content)
                     if inspect.isawaitable(transformed):
                         transformed = await transformed
-                    result[section_id] = {"content": transformed}
+                    result[section_id] = {"content": str(transformed)}
                 except Exception:  # pylint: disable=broad-except
                     result[section_id] = {"content": content}
             else: