ci: add mypy type checking step to pull request workflow

Installs mypy and runs static type checks on the src directory during PR builds.
Ensures type consistency is validated in CI.

Author: plexoos

.github/workflows/build-pull-request.yaml

@@ -32,3 +32,8 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install .
+          pip install mypy
+
+      - name: Type check with mypy
+        run: |
+          mypy src