fix(toolchain): type-annotate test helpers for the full mypy gate

make quality runs mypy over tests/ too; the toolchain test fixtures
(monkeypatch params, the fake-subprocess factories) were missing annotations.
No behavior change.

tests/unit/test_toolchain_flag.py

@@ -2,13 +2,18 @@
 
 from __future__ import annotations
 
+from typing import TYPE_CHECKING
+
 from roboco.config import Settings
 
+if TYPE_CHECKING:
+    import pytest
+
 
 def test_toolchain_match_disabled_by_default() -> None:
     assert Settings().toolchain_match_enabled is False
 
 
-def test_toolchain_match_reads_env(monkeypatch) -> None:
+def test_toolchain_match_reads_env(monkeypatch: pytest.MonkeyPatch) -> None:
     monkeypatch.setenv("ROBOCO_TOOLCHAIN_MATCH_ENABLED", "true")
     assert Settings().toolchain_match_enabled is True