Bug: update MatrixHandler::addConst return type to int (#402)

[pelesh]

• Bug: update MatrixHandler::addConst return type to int

• Doc: Update CHANGELOG.md

Description

External PR #402 merged to a local branch. Passing through CI pipeline before merging to develop.

Closes #395

@MaanasArora

Proposed changes

Correct the return type for MatrixHandler::addConst.

Checklist

• All tests pass. Code tested on
  • CPU backend
  • CUDA backend
  • HIP backend
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows Re::Solve style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.
• I have updated CHANGELOG.md to reflect the changes in this PR. If this is a minor PR that is part of a larger fix already included in the file, state so.