deps: bump @stryker-mutator/core and vitest-runner from 8.7.1 to 9.6.1

Upgrade both Stryker packages together since they share a peer
dependency and must stay on the same major version. Combines the
individual Dependabot PRs #828 and #829 which each failed CI
because they only bumped one side of the pair.

Author: Chris0Jeky