test: address review feedback — fixture lifecycle, test names, ClassVar

- ci-fixture-plugin: call display_manager.clear() before rendering (per
  plugin guidelines — the fixture should model a well-behaved plugin),
  add a class docstring, and document why Pillow is deliberately not
  pinned in its requirements.txt (core dependency; harness installs
  nothing).
- Rename two tests whose names contradicted their assertions:
  test_unparseable_core_version_is_compatible ->
  test_unparseable_core_with_high_floor_is_blocked, and
  test_unreadable_secrets_file... -> test_corrupt_secrets_file...
- Annotate TestGetSchemaProperty.SCHEMA as ClassVar (RUF012).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NohXi78cwsAKtN1sCfxjUh

test/fixtures/plugins/ci-fixture-plugin/manager.py

@@ -16,10 +16,16 @@ from src.plugin_system.base_plugin import BasePlugin
 
 
 class CIFixturePlugin(BasePlugin):
+    """Deterministic CI-only fixture plugin: renders a border + diagonals
+    pattern sized from the display's declared dimensions. Never shipped to
+    devices; exists solely so the plugin safety harness has a real plugin
+    to exercise in CI."""
+
     def update(self) -> None:
         """Nothing to fetch — the render is self-contained."""
 
     def display(self, force_clear: bool = False) -> None:
+        self.display_manager.clear()
         width = self.display_manager.matrix.width
         height = self.display_manager.matrix.height
         border = tuple(self.config.get("border_color", [0, 255, 0]))

test/fixtures/plugins/ci-fixture-plugin/requirements.txt

@@ -1 +1,7 @@
 # No dependencies — the fixture must load in any environment.
+#
+# Pillow is deliberately NOT pinned here even though manager.py imports
+# PIL: it is a core LEDMatrix dependency (see the repo-root
+# requirements.txt), so it is always present wherever the harness runs,
+# and the harness loads plugins with install_deps=False anyway. Pinning
+# it here would only invite a needless pip install during test runs.