EXAMPLES/DEVICE/EP: Added missing const to Buffer::query_mask_buffer. (#1926)

## What?
Added missing `const` to the declaration of `Buffer::query_mask_buffer`.
Added missing `const` to a parameter of `ep_kernels::query_mask_buffer`.

## Why?
To improve the readability of the code, following C++ core guidelines.
Spotted by the PR (https://github.com/ai-dynamo/nixl/pull/1793), need to
reduce the size of the PR, removing unrelated changes.

Signed-off-by: Raul Akhmetshin <rakhmetshin@nvidia.com>