fix: address review feedback for approx_top_k

- Replace HashMap<Vec<u8>, usize> with hashbrown::HashTable<(u64, usize)>
  to eliminate key cloning, double-hashing, and O(n) rebuild allocations
- Fix merge() to use Parallel Space-Saving m1/m2 algorithm (min counter
  count correction) matching ClickHouse's SpaceSaving::merge()
- Fix serialize() to select top-N counters and fold evicted ones into
  alpha_map when requested_capacity < counters.len() < target_capacity
- Harden from_bytes() with validation: requested_capacity > 0,
  num_counters <= requested_capacity, alpha_map_size is power-of-two,
  and overflow-safe bounds checking throughout
- Replace all raw += on count/error/alpha with saturating_add
- Track item_heap_bytes incrementally for O(1) size() accounting
- Pre-allocate serialize() output buffer
- Add TIMEZONE_WILDCARD timestamp variants to signature
- Remove unused k and data_type from state_fields/state/merge_batch
- Replace all manual DataFusionError construction with exec_err!/plan_err!
- Replace .unwrap() downcasts in update_batch with proper error returns
- Fix test_accumulator_large_utf8_input to use DataType::LargeUtf8
- Update #[user_doc] to document approximate counts, NULL handling,
  supported types, and return shape
- Use with_timezone() instead of arrow::compute::cast for timestamps
- Adapt to upstream API changes (remove as_any from AggregateUDFImpl,
  use direct downcast_ref for Literal)

Author: sesteves

Cargo.lock

@@ -50,6 +50,7 @@ datafusion-functions-aggregate-common = { workspace = true }
 datafusion-macros = { workspace = true }
 datafusion-physical-expr = { workspace = true }
 datafusion-physical-expr-common = { workspace = true }
+hashbrown = { workspace = true }
 foldhash = "0.2"
 half = { workspace = true }
 log = { workspace = true }