feat: redesign code interpreter with structured Execution model

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>

.gitignore

@@ -175,3 +175,5 @@ cython_debug/
 .pypirc
 
 CODE_EXECUTION.md
+
+docs/