fix(P0 rust): assistant-terminal rejection fired on OpenCode's normal mid-turn continuation shape — prev_response_completed_at_ms marks the previous STEP's EOF, not a finished turn, so every tool-using rust-mode turn died once the new adapter began forwarding it. Serve real-content tails untouched (provider serializer owns wire termination); keep contentless re-anchor + newest-synthetic preservation.

Co-Authored-By: Alfonso <alfonso@cortexkit.io>