fix(ci): pop PYTHONHOME in nox before running pytest to fix Windows CI

On Windows, uv run sets PYTHONHOME to the managed Python 3.14 install
directory when starting nox. This leaks into nox sub-sessions so pytest
processes for Python 3.11–3.13 load Python 3.14's stdlib and crash.

Fix by clearing PYTHONHOME in os.environ at the start of _run_test_suite
in noxfile.py, before any pytest subprocess is spawned. Also removes the
now-unnecessary `unset PYTHONHOME` from the composite action (it was a
no-op: uv run re-injects PYTHONHOME regardless).

Author: olivermeyer

noxfile.py

@@ -893,6 +893,12 @@ def _run_test_suite(session: nox.Session, marker: str = "", cov_append: bool = F
         marker: Pytest marker expression
         cov_append: Whether to append to existing coverage data
     """
+    # On Windows, uv run sets PYTHONHOME to the managed Python 3.14 install dir when it
+    # starts nox. This leaks into pytest sub-processes for older Python versions (3.11-3.13)
+    # and causes them to load Python 3.14's stdlib, crashing at startup. Clear it here so
+    # all pytest sub-processes inherit a clean environment. No-op on Linux/macOS where uv
+    # does not set PYTHONHOME.
+    os.environ.pop("PYTHONHOME", None)
     _setup_venv(session)
 
     posargs = session.posargs[:]