Replace flat CodeResult with a proper model hierarchy: Execution
(top-level), Result (per-output with typed MIME fields), Logs
(stdout/stderr as lists), and ExecutionError (structured
name/value/traceback). Handle display_data messages for rich output,
add streaming callbacks (on_result, on_stdout, on_stderr, on_error),
and remove the misleading stdout-to-text fallback.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
