Make prefix filter AFL fuzzer comprehensive

Close the remaining coverage gaps:

- MVCC snapshot reads: TakeSnapshot captures the current seqno, then
  SnapshotGet and SnapshotPrefixScan read at the frozen point while new
  writes continue. Tests version pinning + prefix filter interaction.

- WeakDelete: calls remove_weak() so AFL can explore weak tombstone GC
  combined with prefix filter correctness during compaction.

- Extractor change on reopen: ReopenNewExtractor swaps to a different
  extractor, testing the prefix_filter_allowed() compatibility gating
  where old tables keep their original extractor metadata.

- Partitioned filter control: FilterPartitioningChoice lets AFL force
  partitioned filters on all levels (including L0 flushes), not just
  the default L3+. This is the code path where we found the empty
  tli_handles panic.

- FirstKV / LastKV: unbounded iteration endpoints that should never be
  affected by prefix filter pruning.

- ContainsKey: same point-read path as Get, now explicitly tested.

- Empty prefix: ClusteredPrefix length lowered to 0..=3, where len=0
  maps to an unbounded full scan.

Author: zaidoon1