Create a mixed-version scenario where an 8.3 cluster runs the existing $vectorSearch extension with legacy optimization hooks, then upgrade binaries and the extension together to 9.0 with the new optimization API and featureFlagExtensionsOptimizations enabled while running queries continuously, verifying no correctness regressions and that limit, sort-elimination, and batchsize-tuning optimizations are active post-upgrade (preferably using the real $vectorSearch extension, even if the test is manual or Evergreen-optional).