mason: keep latest assistant reasoning native

Replay the complete native OpenCode part vector whenever the latest assistant carries signed reasoning, because provider projection can regroup even suffix mutations ahead of thinking and invalidate its bytes or position. This intentionally changes the prior sibling-tool-mutation assertion to full native replay for that exempt message.

Refresh the generator-owned ASTRO rejection fixture and align the plugin golden gate with the TypeScript encoder output.

Co-authored-by: Alfonso <alfonso-magic-context@users.noreply.github.com>