In order to fix some namespace-related errors on vector search tests running on extension vector search enabled suites, setting  TestData.pinToSingleMongos to true was used to avoid issues with stale routers. Doing so doesn't fix the underlying issue and instead hides it (ex: see this bug SERVER-119436).

We should audit all other uses of TestData.pinToSingleMongos = true; in our tests (ex: like here) and remove them and fix any errors that arise. This cleanup effort should also target removing  unnecessary uses of getSiblingDb() where applicable.