Fix turn-usage keying: idempotent per request turn id, not messageId

Live smoke exposed two bugs in the turn usage log:
- onFinish messages carry no ids (withMessageIds fills copies inside
  persistTurn), so fresh turns keyed messageId "" — provenance lost.
- The SDK coalesces continuation turns into ONE response message (a whole
  probe conversation lands in a single assistant message), so per-messageId
  idempotency swallowed every continuation turn's usage.

TurnUsage gains id: string (unique per request, the append idempotency
key); messageId stays as transcript provenance, taken from the id-filled
list. appendTurnUsage dedupes by id; continuation turns with the same
messageId each keep their usage.

tests/render-stats-bar.test.tsx

@@ -15,6 +15,7 @@ function step(inputTokens: number, outputTokens: number, over: Record<string, nu
 
 function turn(over: Partial<TurnUsage> = {}): TurnUsage {
 	return {
+		id: "t1",
 		messageId: "m1",
 		at: "2026-09-09T12:00:00Z",
 		model: "z-ai/glm-5.3-flash",