Completes the session-binding class on the gateway surface (#55578),
matching the TUI rules:
1. Fail-closed pinning: switch_session() re-opens ended sessions, so
pinning a completion to a spawning session that has since ENDED
(user /new, closed rotation) would resurrect a conversation the user
explicitly ended and inject into it. The injection path now checks
the pinned row's ended_at first and drops the injection with a
WARNING when the spawning session is dead or unknown - the result
stays in the delegation records.
2. /new ends the old conversation's delegations: _handle_reset_command
calls interrupt_for_session() with the expiring durable session id
(matching the parent_session_id pin stamped at dispatch) plus the
routing key as fallback, so a reset can't leave dangling subagents
whose completions have no live owner.
interrupt_for_session() gains the parent_session_id selector because a
gateway chat's session_key (the platform conversation key) survives a
reset while the session id rotates - key-based matching alone could
never sever a gateway conversation's delegations.
