perf(brute_force): skip csr_to_coo on inner-product filtered search (#2128)

## Summary

Small perf cleanup for the filtered brute-force CSR path in `knn_brute_force.cuh`.

- When sparsity ≥ 0.9, filtered brute-force takes the sparse CSR / `masked_matmul` path. As part of setup it builds a per-nonzero `rows` array via `raft::sparse::convert::csr_to_coo`.
- That `rows` array is only consumed by `cuvs::neighbors::detail::epilogue_on_csr`, which only runs for `L2Expanded` / `L2SqrtExpanded` / `CosineExpanded` (it combines masked inner products with precomputed norms).
- For `InnerProduct`, the epilogue is skipped — so the allocation and the `csr_to_coo` kernel launch were dead work on every IP-metric filtered search hitting the CSR path.

This PR hoists the `rmm::device_uvector<IdxT> rows(...)` allocation and the `csr_to_coo` call inside the L2/Cosine branch, next to their only consumer.

## What changes

- No behavior change for `L2Expanded`, `L2SqrtExpanded`, `CosineExpanded` — same kernels, same order.
- For `InnerProduct` filtered searches that hit the sparse CSR branch: saves one device allocation of `nnz * sizeof(IdxT)` and one kernel launch + write pass per call.

## Test plan

- [ ] Existing filtered brute-force tests pass (`BRUTE_FORCE_TEST`, prefiltered variants) for `InnerProduct`, `L2Expanded`, `L2SqrtExpanded`, `CosineExpanded`.
- [x] No diff in output for L2 / Cosine vs. base.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Authors:
  - Max Buckley (https://github.com/maxwbuckley)
  - Micka (https://github.com/lowener)
  - Anupam (https://github.com/aamijar)

Approvers:
  - Micka (https://github.com/lowener)

URL: #2128

Author: maxwbuckley

cpp/src/neighbors/detail/knn_brute_force.cuh

@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: Copyright (c) 2023-2026, NVIDIA CORPORATION.
+ * SPDX-FileCopyrightText: Copyright (c) 2023-2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
  * SPDX-License-Identifier: Apache-2.0
  */
 
@@ -671,13 +671,7 @@ void brute_force_search_filtered(
 
     // create filter csr view
     auto compressed_csr_view = csr.structure_view();
-    rmm::device_uvector<IdxT> rows(compressed_csr_view.get_nnz(), stream);
-    raft::sparse::convert::csr_to_coo(compressed_csr_view.get_indptr().data(),
-                                      compressed_csr_view.get_n_rows(),
-                                      rows.data(),
-                                      compressed_csr_view.get_nnz(),
-                                      stream);
-    auto dataset_view = raft::make_device_matrix_view<const T, IdxT, raft::row_major>(
+    auto dataset_view        = raft::make_device_matrix_view<const T, IdxT, raft::row_major>(
       idx.dataset().data_handle(), n_dataset, dim);
 
     auto csr_view = raft::make_device_csr_matrix_view<DistanceT, IdxT, IdxT, IdxT>(
@@ -714,6 +708,12 @@ void brute_force_search_filtered(
             query_norms_->view());
         }
       }
+      rmm::device_uvector<IdxT> rows(compressed_csr_view.get_nnz(), stream);
+      raft::sparse::convert::csr_to_coo(compressed_csr_view.get_indptr().data(),
+                                        compressed_csr_view.get_n_rows(),
+                                        rows.data(),
+                                        compressed_csr_view.get_nnz(),
+                                        stream);
       cuvs::neighbors::detail::epilogue_on_csr(
         res,
         csr.get_elements().data(),