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/web_interface/test_api_v3_helpers.py

@@ -11,6 +11,7 @@ characterized quirks marked below.
 
 import sys
 from pathlib import Path
+from typing import Any, ClassVar, Dict
 
 import pytest
 
@@ -153,7 +154,7 @@ class TestParseFormValue:
 
 
 class TestGetSchemaProperty:
-    SCHEMA = {
+    SCHEMA: ClassVar[Dict[str, Any]] = {
         "properties": {
             "brightness": {"type": "integer"},
             "customization": {