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") {

src/gateway/server-chat.ts

@@ -346,6 +346,7 @@ export function createAgentEventHandler({
     seq: number,
     jobState: "done" | "error",
     error?: unknown,
+    stopReason?: string,
   ) => {
     const bufferedText = stripInlineDirectiveTagsForDisplay(
       chatRunState.buffers.get(clientRunId) ?? "",
@@ -399,6 +400,7 @@ export function createAgentEventHandler({
         sessionKey,
         seq,
         state: "final" as const,
+        ...(stopReason && { stopReason }),
         message:
           text && !shouldSuppressSilent
             ? {
@@ -512,6 +514,8 @@ export function createAgentEventHandler({
       if (!isAborted && evt.stream === "assistant" && typeof evt.data?.text === "string") {
         emitChatDelta(sessionKey, clientRunId, evt.runId, evt.seq, evt.data.text);
       } else if (!isAborted && (lifecyclePhase === "end" || lifecyclePhase === "error")) {
+        const evtStopReason =
+          typeof evt.data?.stopReason === "string" ? evt.data.stopReason : undefined;
         if (chatLink) {
           const finished = chatRunState.registry.shift(evt.runId);
           if (!finished) {
@@ -525,6 +529,7 @@ export function createAgentEventHandler({
             evt.seq,
             lifecyclePhase === "error" ? "error" : "done",
             evt.data?.error,
+            evtStopReason,
           );
         } else {
           emitChatFinal(
@@ -534,6 +539,7 @@ export function createAgentEventHandler({
             evt.seq,
             lifecyclePhase === "error" ? "error" : "done",
             evt.data?.error,
+            evtStopReason,
           );
         }
       } else if (isAborted && (lifecyclePhase === "end" || lifecyclePhase === "error")) {