Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@cpp/src/neighbors/detail/cagra/cagra_helpers.cpp`:
- Around line 184-208: The extend_gpu_mem variable is being calculated when
add_data_on_build is true but is not being included in the total_dev
calculation. Fix this by adding extend_gpu_mem to the std::max initializer list
that computes total_dev alongside kmeans_gpu_mem, search_phase_dev, and
gpu_workspace_size to ensure the extend-phase GPU memory requirement is properly
accounted for in the final device memory estimate.
- Around line 159-239: The `ivf_pq_build_mem_usage` function has a broken
control flow where the return statement is inside the debug logging condition
block, causing a compile error when debug logging is disabled. Additionally, the
code references an undefined variable `pq_params` when the actual parameter is
`params`. To fix this, move the return statement that returns
`std::make_pair(total_host, total_dev)` to the end of the function after the
closing brace of the debug logging if-block, change
`pq_params.build_params.add_data_on_build` to
`params.build_params.add_data_on_build`, and ensure all code after the if-block
has correct indentation so the function properly exits regardless of whether
debug logging is enabled.

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@cpp/src/neighbors/detail/cagra/cagra_helpers.cpp`:
- Around line 87-99: The if block in the optimize_workspace_size function that
checks raft::default_logger().should_log(rapids_logger::level_enum::debug) is
opened with a brace but never closed, causing the total_host, total_host_fixed,
total_dev, total_dev_fixed variable declarations and the return statement to be
incorrectly nested inside the if block. Add a closing brace immediately after
the debug_host_size calculation (after the hist comment) and before the size_t
total_host declaration to properly close the if block and allow the function to
execute correctly in both debug and non-debug code paths.
- Around line 101-136: The ivf_pq_extend_mem_usage function lacks validation for
the pq_dim parameter, which can default to 0 when using the default constructor
and will cause division by zero when used in raft::round_up_safe and in the
code_bytes_per_vec calculation. Add a RAFT_EXPECTS check at the beginning of the
function to ensure params.build_params.pq_dim is non-zero before it is used in
the rot_dim assignment and the code_bytes_per_vec calculation.

---

Duplicate comments:
In `@cpp/src/neighbors/detail/cagra/cagra_helpers.cpp`:
- Line 185: Replace the undefined variable `pq_params` with the correct
parameter name `params` in the condition at line 185 where
`pq_params.build_params.add_data_on_build` is being accessed. The function
parameter is named `params` (defined on line 143), not `pq_params`, so update
the reference to use the correct variable name to ensure the code compiles.