fix(gateway+acp): thread stopReason through final event to ACP bridge (#24867)

Complete the stop reason propagation chain so ACP clients can
distinguish end_turn from max_tokens:

- server-chat.ts: emitChatFinal accepts optional stopReason param,
  includes it in the final payload, reads it from lifecycle event data
- translator.ts: read stopReason from the final payload instead of
  hardcoding end_turn

Chain: LLM API → run.ts (meta.stopReason) → agent.ts (lifecycle event)
→ server-chat.ts (final payload) → ACP translator (PromptResponse)

src/acp/translator.ts

@@ -423,7 +423,9 @@ export class AcpGatewayAgent implements Agent {
     }
 
     if (state === "final") {
-      this.finishPrompt(pending.sessionId, pending, "end_turn");
+      const rawStopReason = payload.stopReason as string | undefined;
+      const stopReason: StopReason = rawStopReason === "max_tokens" ? "max_tokens" : "end_turn";
+      this.finishPrompt(pending.sessionId, pending, stopReason);
       return;
     }
     if (state === "aborted") {