fix(svs_flat): accept all-clear bitset as unfiltered search

Before: any non-empty BitsetView passed to SvsFlatIndexNode::Search()
caused an immediate `not_implemented` return, even when every bit was
zero (i.e. no vectors filtered out). Callers that pre-allocate a bitset
for shape consistency -- e.g. the vecTool nightly harness always passes
a bitset whose size matches the base set but leaves it all-clear for
plain kNN -- would see every SVS_FLAT search fail at the first query.

After: reject only when the bitset is both non-empty AND has at least
one bit set. All-clear bitsets fall through to the non-bitset search
path, which is the correct semantic for "no filtering requested".

`BitsetView::count()` (include/knowhere/bitsetview.h:50) returns the
number of filtered-out bits / ids, so the added `&& bitset.count() > 0`
is cheap and doesn't scan the bitmap.

Also register raw `SVS_VAMANA` in the SVS registration block so the
factory can construct it, matching the existing `Type()` implementation
and the LVQ / LeanVec registrations in the same file.

No regression on the actual-filter case: when the caller does have bits
set, we still surface the same not_implemented error with the same
message string, matching the current behavior.

Fixes one of the class of failures seen when running the vecTool
newNightly L2 SVS smoke benchmark (all 6 SVS_FLAT groups across
cohere_1m + qwen3vl_200k x {fp32, fp16, bf16} previously failed
identically at search stage; HNSW siblings on the same group IDs
passed). See zilliztech/vecTool#21 for the upstream discussion.

Signed-off-by: jamesgao-jpg <james.gao@zilliz.com>

Author: jamesgao-jpg

src/index/svs/svs_flat.cc

@@ -76,7 +76,7 @@ class SvsFlatIndexNode : public IndexNode {
             return expected<DataSetPtr>::Err(Status::empty_index, "index not loaded");
         }
 
-        if (!bitset.empty()) {
+        if (!bitset.empty() && bitset.count() > 0) {
             return expected<DataSetPtr>::Err(Status::not_implemented, "SVS Flat does not support bitset filtering");
         }
 

src/index/svs/svs_vamana.cc

@@ -519,6 +519,8 @@ class SvsVamanaLeanVecIndexNode : public SvsVamanaIndexNode<DataType> {
 
 // temporarily removed `MMAP` until it's fully honored by SVS
 
+KNOWHERE_MOCK_REGISTER_DENSE_FLOAT_ALL_GLOBAL(SVS_VAMANA, SvsVamanaIndexNode, knowhere::feature::NONE)
+
 KNOWHERE_MOCK_REGISTER_DENSE_FLOAT_ALL_GLOBAL(SVS_VAMANA_LVQ, SvsVamanaLvqIndexNode, knowhere::feature::NONE)
 
 KNOWHERE_MOCK_REGISTER_DENSE_FLOAT_ALL_GLOBAL(SVS_VAMANA_LEANVEC, SvsVamanaLeanVecIndexNode, knowhere::feature::NONE)