test(native-chat): stamp isReplay on the second fake Claude echo too

`src/main/runtime/claude-structured-session-integration.test.ts` has its own
fake connection echoing `{ ...message, uuid: 'user-1' }` without `isReplay`, so
the send waited out its whole ack budget and settled `unknown`. CI red; missed
locally because the previous run covered only src/main/claude and
src/main/native-chat.

Second instance of the hazard the sibling comment already names: a double that
does not stamp like the real binary. Swept the tree - these two fakes are the
only ones that echo a user frame.

Found by the review loop on #15657.

src/main/runtime/claude-structured-session-integration.test.ts

@@ -82,7 +82,9 @@ function fakeClaude() {
       send: async (message) => {
         connection.sent.push(message)
         if (message.type === 'user') {
-          handlers.onMessage?.({ ...message, uuid: 'user-1' })
+          // The real CLI stamps `isReplay` on the echo `--replay-user-messages`
+          // produces; a double that omits it does not exercise the send path.
+          handlers.onMessage?.({ ...message, uuid: 'user-1', isReplay: true })
         }
       },
       respond: async (requestId, response) => {