test(subagent): preserve the flush result in the failure fixture

The gated final-flush fixture awaited the real flush but dropped its boolean return value. The regression ran correctly under Vitest, but the full Host build rejected the mock against Sessions.flush Promise<boolean>.

Return the real flush result after the gate. This retains the observation order and matches the service signature without a cast or altered production behavior.

Validation: targeted terminal-capture-failure regression passes; the complete build is rerun after this typing correction.

packages/subagent/subagent/tests/continuation.spec.ts

@@ -2445,7 +2445,7 @@ describe('continuable review regressions', () => {
         flushing.resolve(undefined)
         await releaseFlush.promise
       }
-      await flush(session)
+      return flush(session)
     })
     const failure = new Error('capture failed')
     const result = started.result.catch((error: unknown) => error)