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/items/pytest/test.tum

@@ -0,0 +1,17 @@
+- pytest:
+    name: Pytest item
+    test_file: {{include_directory}}/test_cases.py
+    key: $(test)_PASS
+    test_method:
+        - test_01_pass
+        - test_02_pass
+        - test_05_param
+
+- pytest:
+    name: Pytest item
+    test_file: {{include_directory}}/test_cases.py
+    key: $(test)_FAIL
+    test_method:
+        - test_01_pass
+        - test_03_fail
+        - test_04_skip