pytest-xdist: scope='session' fixtures run once per worker; share via FileLock

Tested with pytest 9.2.0, pytest-xdist 3.8.0 and CPython 3.13. With -n 3, a scope="session" fixture's setup ran 3 times, once in each worker process that received tests (once with -n 0), so an expensive download, DB build or server start is repeated per worker, and workers can race on the same external resource. To do it once per run, key a file on the shared base temp dir and guard it with filelock: root = tmp_path_factory.getbasetemp().parent; fn = root / 'data.json'; with FileLock(str(fn) + '.lock'): load fn if it exists, else build it and write it. Branch on xdist's worker_id fixture: it is "master" under -n 0, so build directly in that case. The fixture is provided by the plugin, so with -p no:xdist (or xdist not installed) every test errors at setup. In my test, with -n 3 the build function ran exactly once, and all 12 tests saw the same data. Teardown is not coordinated by this pattern, because each worker's fixture finalizer runs on its own, so don't delete the shared resource there.