feat: add stop_reason field to ResultMessage

[qing-ant]

Adds stop_reason: str | None to ResultMessage, matching the TypeScript SDK.

The field carries the API's stop reason ("end_turn", "max_tokens", etc.) through to the result message. Backward-compatible: defaults to None and the parser uses data.get("stop_reason"), so older CLI output without the field produces None rather than failing.

• Add stop_reason: str | None field to ResultMessage

Test plan

• Unit: parser tests for explicit value, explicit null → None, and absent field → None
• E2E: verified against CLI v2.1.64-dev — SDK parsing correct (CLI-side population tracked separately)