End a TEXT conversation (status -> completed).

Valid only for channel="text" (409 otherwise). Finalizes the conversation — computes duration, writes a gated summary, and runs the agent's post-conversation workflow. Idempotent: ending an already-closed conversation returns 200 with the existing record (no re-close).

transferred counts as already-closed here. A handed-off conversation is finalized by close_handoff, which sets that status and has already run the post-conversation workflow; treating it as "not completed" would turn the widget's ordinary end-of-chat call into a 409, and treating it as endable would run the workflow a second time and overwrite the bot-phase duration with wall clock.