Add model-controlled truncation for code mode results

[pakrym-oai]

Summary

• document that @openai/code_mode exposes set_max_output_tokens_per_exec_call and that code_mode truncates the final Rust-side output when the budget is exceeded
• enforce the configured budget in the Rust tool runner, reusing truncation helpers so text-only outputs follow the unified-exec wrapper and mixed outputs still fit within the limit
• ensure the new behavior is covered by a code-mode integration test and string spec update

Testing

• Not run (not requested)