Use pytest-forked to isolate MPI state

Add `--forked` flag to `run_integrated_tests.sh` to isolate C++/MPI state between tests, preventing OpenMPI 5 double-initialization crashes.

Author: tomchapman

requirements.txt

@@ -8,3 +8,4 @@ boutdata>=0.3.0
 zoidberg>=0.2.2
 pytest
 pytest-xdist
+pytest-forked

tests/run_integrated_tests.sh

@@ -7,6 +7,6 @@ export PYTHONPATH="@PROJECT_BINARY_DIR@/tools/pylib:@PROJECT_SOURCE_DIR@/tools/p
 # Pre-build the project to prevent concurrent CMake race conditions
 cmake --build "$PROJECT_ROOT"
 
-# Use the Python executable that CMake discovered during configuration
-"@Python3_EXECUTABLE@" -m pytest -m "not serial" --cache-clear -n auto --dist=loadgroup -q "$PROJECT_ROOT/tests/integrated" "$@"
-"@Python3_EXECUTABLE@" -m pytest -m serial "$PROJECT_ROOT/tests/integrated" "$@"
+# Use --forked to isolate C++/MPI state for every single test
+"@Python3_EXECUTABLE@" -m pytest --forked -m "not serial" --cache-clear -n auto --dist=loadgroup -q "$PROJECT_ROOT/tests/integrated" "$@"
+"@Python3_EXECUTABLE@" -m pytest --forked -m serial "$PROJECT_ROOT/tests/integrated" "$@"