Add BONSAI_TEST_ARGS env-var fallback to runpytest.py

PowerShell and some wrapper scripts on Windows occasionally strip
or reorder the `--` separator before Blender sees it, dropping the
pytest args into Blender's positional file-load slot ("File format
is not supported"). The env var carries the same args via a
shell-evaluation-free channel. Default `--` path is byte-identical
to the pre-change behaviour.

Generated with the assistance of an AI coding tool.

src/bonsai/runpytest.py

@@ -17,18 +17,46 @@
 # along with Bonsai.  If not, see <http://www.gnu.org/licenses/>.
 
 """
-Requires pytest installed under blender
+Requires pytest installed under blender.
 
-Usage: `blender -b -P runpytest.py -- ARGS`
+Usage:
+    blender -b -P runpytest.py -- ARGS
+
+Alternative (when the calling shell strips or reorders the ``--`` separator
+before it reaches Blender — observed with some PowerShell / wrapper-script
+invocations on Windows): pass the same pytest args via the
+``BONSAI_TEST_ARGS`` environment variable as a single shell-quoted string
+and invoke without ``--``::
+
+    $env:BONSAI_TEST_ARGS = "test/bim/ -x -q"
+    blender -b -P runpytest.py
 """
 
+import os
+import shlex
 import sys
 
 import pytest
 
 argv = [__file__]
 
-if "--" in sys.argv:
+env_args = os.environ.get("BONSAI_TEST_ARGS", "")
+if env_args:
+    # POSIX-style quoting works on all three OSes — env var values are
+    # literal strings (no shell evaluation when Python reads them), and
+    # POSIX quoting (``'foo "bar baz" qux'`` → three tokens, quotes stripped)
+    # matches what most docs and examples use.
+    argv += shlex.split(env_args)
+    # On the env-var path the args never appear in Blender's argv at all,
+    # so any pytest plugin that reads ``sys.argv`` directly (instead of
+    # going through pytest's API) would otherwise see only Blender's own
+    # ``-b -P runpytest.py`` and miss the test args entirely. Shadow argv
+    # so those plugins see the pytest-shaped view they expect.
+    sys.argv = list(argv)
+elif "--" in sys.argv:
+    # The traditional path: Blender forwards everything after ``--`` to the
+    # script via ``sys.argv``. ``sys.argv`` is deliberately left as Blender
+    # set it — pre-existing behavior, preserved.
     i = sys.argv.index("--")
     argv += sys.argv[i + 1 :]