feat(items): add pytest test item

Run a user pytest file as a testium item, surfacing each collected test
as a child with its own PASS/FAIL/SKIP, duration and failure message.

Mirrors the unittest item but runs pytest in a subprocess on the host
interpreter (bins.python_bin(), like py_func/lua_func) so it works across
every packaging channel. A stdlib-only pytest plugin streams collected
node-ids and per-test results over stdout via sentinels; the parent parses
them live. Params: test_file, test_method. stop_on_failure maps to -x;
disabled children are reported NORUN without running.

Wiring: TYPE_PYTEST / TYPE_PYTEST_STEP constants, test_init registration,
self-loading branch in test_set, GUI tree icon. Schema/LSP pick it up
automatically from the declarative PARAMS.

Validation: test/validation/items/pytest/ (validation venv now installs
pytest).

WIP: paused mid-feature (DESIGN.md documented; manual section pending).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/validation/run.sh

@@ -21,7 +21,7 @@
 # even Flatpak reaches it via flatpak-spawn --host. The validation venv
 # is created with --system-site-packages so existing system packages
 # (PySide6, lxml, ...) stay visible, then junit-xml is pip-installed
-# for post_execution.py.
+# for post_execution.py and pytest for the `pytest` item.
 #
 # The report file is suffixed with the mode (e.g. validation-flatpak.sqlite)
 # so consecutive runs in different modes don't overwrite each other.
@@ -73,7 +73,7 @@ if [ ! -d "$VENV_DIR" ]; then
     echo "Creating validation venv at $VENV_DIR"
     python3 -m venv --system-site-packages "$VENV_DIR"
     "$VENV_DIR/bin/pip" install --quiet --upgrade pip
-    "$VENV_DIR/bin/pip" install --quiet junit-xml
+    "$VENV_DIR/bin/pip" install --quiet junit-xml pytest
 fi
 VENV_PYTHON="$VENV_DIR/bin/python3"