Make the Guardiann test helpers reusable as test fixtures

[normen662]

Moves the reusable Guardiann test code into the fdb-extensions test fixtures, so that it is published with them (see #4676) and can be reused outside this build. Existing tests use the moved code; their behavior is unchanged.

• guardiann/TestHelpers becomes the public GuardiannTestHelpers fixture (batch insert/delete, sampling, recall and ordered-by-distance checks, the test listeners). countUnderReplicatedPrimaries moves into its only user, ReassignConvergenceTest.
• ConfigRecommendation moves into the fixtures and becomes public.
• Minimal generalizations: the dynamic recall check takes the metric of its brute-force ground truth (existing callers pass EUCLIDEAN_SQUARE_METRIC, which computes exactly the previous l2SquaredDistance), and the recall checks accept any searcher (BiFunction<RealVector, Integer, List<Tuple>>); the Guardiann overloads delegate to them.
• The reusable parts of SiftTest become static helpers: interleavedChurnSchedule and shuffledDeleteBatches (the insert/delete schedules, with the same use of the random generator as before), and the assertOnlyPrimaryKeysRemain, assertDrainedToSingleEmptyCluster and describeStructure structure checks in GuardiannStructureAsserts. SiftTest keeps its tests and loops, which now use them.

Verified: :fdb-extensions:check passes (including scalarFallbackTest); the recall, quality and checkpoint log lines of all six SiftTest tests (CI and -Ptests.nightly variants) are byte-identical to main for the same seed.