fix: prevent tests from touching real Steam files

Tests for pick_next_game were calling uninstall_other_games and
state.save against real filesystem paths, deleting installed games
and overwriting the user's state.json whenever tests or pre-commit
ran.

- Add conftest.py safety net that redirects STEAMAPPS_PATH,
  CONFIG_DIR, STATE_FILE, SNAPSHOT_FILE, CONFIG_FILE, and
  HOSTS_FILE to tmp_path in all steam_backlog_enforcer tests
- Add missing uninstall_other_games mock to 4 tests in
  test_scanning.py (test_picks_shortest, test_skips_finished,
  test_unknown_hours, test_picks_game_no_hours)

steam_backlog_enforcer/tests/conftest.py

@@ -0,0 +1,70 @@
+"""Safety conftest: prevent tests from touching real Steam/config files.
+
+Redirects all filesystem paths used by the steam_backlog_enforcer package
+to temporary directories.  This stops tests from accidentally:
+  - Deleting real game files via uninstall_other_games / uninstall_game
+  - Overwriting ~/.config/steam_backlog_enforcer/state.json (losing the
+    user's current assignment)
+  - Reading real appmanifest files from ~/.local/share/Steam/steamapps
+  - Modifying /etc/hosts via the store blocker
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+from unittest.mock import patch
+
+import pytest
+
+if TYPE_CHECKING:
+    from collections.abc import Iterator
+    from pathlib import Path
+
+
+@pytest.fixture(autouse=True)
+def _isolate_filesystem(tmp_path: Path) -> Iterator[None]:
+    """Redirect all real filesystem paths to a temporary directory.
+
+    Individual tests that also patch these paths will simply override
+    this fixture's patches for the duration of their own ``with`` block.
+    """
+    fake_config = tmp_path / "config"
+    fake_config.mkdir()
+    fake_steamapps = tmp_path / "steamapps"
+    fake_steamapps.mkdir()
+    fake_hosts = tmp_path / "hosts"
+
+    with (
+        # Config / state / snapshot paths (used by State.save, Config.save, etc.)
+        patch(
+            "python_pkg.steam_backlog_enforcer.config.CONFIG_DIR",
+            fake_config,
+        ),
+        patch(
+            "python_pkg.steam_backlog_enforcer.config.CONFIG_FILE",
+            fake_config / "config.json",
+        ),
+        patch(
+            "python_pkg.steam_backlog_enforcer.config.STATE_FILE",
+            fake_config / "state.json",
+        ),
+        patch(
+            "python_pkg.steam_backlog_enforcer.config.SNAPSHOT_FILE",
+            fake_config / "snapshot.json",
+        ),
+        # Steam game manifests / install dirs
+        patch(
+            "python_pkg.steam_backlog_enforcer.game_install.STEAMAPPS_PATH",
+            fake_steamapps,
+        ),
+        # /etc/hosts (store blocker)
+        patch(
+            "python_pkg.steam_backlog_enforcer.store_blocker.HOSTS_FILE",
+            fake_hosts,
+        ),
+        patch(
+            "python_pkg.steam_backlog_enforcer.config.HOSTS_FILE",
+            fake_hosts,
+        ),
+    ):
+        yield

steam_backlog_enforcer/tests/test_scanning.py

@@ -228,6 +228,10 @@ class TestPickNextGame:
                 "python_pkg.steam_backlog_enforcer.scanning.is_game_installed",
                 return_value=True,
             ),
+            patch(
+                "python_pkg.steam_backlog_enforcer.scanning.uninstall_other_games",
+                return_value=0,
+            ),
         ):
             pick_next_game([g1, g2], state, config)
             assert state.current_app_id == 2
@@ -255,6 +259,10 @@ class TestPickNextGame:
                 "python_pkg.steam_backlog_enforcer.scanning.is_game_installed",
                 return_value=True,
             ),
+            patch(
+                "python_pkg.steam_backlog_enforcer.scanning.uninstall_other_games",
+                return_value=0,
+            ),
         ):
             pick_next_game([g1, g2], state, config)
             assert state.current_app_id == 2
@@ -331,6 +339,10 @@ class TestPickNextGame:
                 "python_pkg.steam_backlog_enforcer.scanning.is_game_installed",
                 return_value=True,
             ),
+            patch(
+                "python_pkg.steam_backlog_enforcer.scanning.uninstall_other_games",
+                return_value=0,
+            ),
         ):
             pick_next_game([g1, g2], state, config)
             assert state.current_app_id == 2
@@ -350,6 +362,10 @@ class TestPickNextGame:
                 "python_pkg.steam_backlog_enforcer.scanning.is_game_installed",
                 return_value=True,
             ),
+            patch(
+                "python_pkg.steam_backlog_enforcer.scanning.uninstall_other_games",
+                return_value=0,
+            ),
         ):
             pick_next_game([g1], state, config)
             assert state.current_app_id == 1