fix: address PR review threads - test assertions, ruff violations, format_kb consistency

Resolves review threads:
- PRRT_kwDOO6Hdxs5vNroH (Thread 36): tests now verify generic ToolError message
  instead of raw exception text (security: no sensitive data in user-facing errors)
- PRRT_kwDOO6Hdxs5vNuYg (Thread 14): format_kb KB branch now uses :.2f like all
  other branches (consistency fix)
- I001/F841/PERF401: fix ruff violations in http_layer, integration, safety tests

Changes:
- tests/test_array.py: match "Failed to execute array/parity_status" (not raw error)
- tests/test_keys.py: match "Failed to execute keys/list" (not raw error)
- tests/test_notifications.py: match "Failed to execute notifications/overview" (not raw error)
- tests/test_storage.py: update format_kb assertion to "512.00 KB" (:.2f format)
- tests/http_layer/test_request_construction.py: remove unused result var (F841)
  + fix import sort (I001)
- tests/safety/test_destructive_guards.py: use list.extend (PERF401) + fix import sort
- unraid_mcp/core/utils.py: format_kb returns f"{k:.2f} KB" for sub-MB values

Co-authored-by: @coderabbitai
Co-authored-by: @cubic-dev-ai
Co-authored-by: @copilot-pull-request-reviewer

unraid_mcp/core/utils.py

@@ -13,6 +13,7 @@ def safe_get(data: dict[str, Any], *keys: str, default: Any = None) -> Any:
 
     Returns:
         The value at the end of the key chain, or default if unreachable.
+        Explicit ``None`` values at the final key also return ``default``.
     """
     current = data
     for key in keys:
@@ -65,4 +66,4 @@ def format_kb(k: Any) -> str:
         return f"{k / (1024 * 1024):.2f} GB"
     if k >= 1024:
         return f"{k / 1024:.2f} MB"
-    return f"{k} KB"
+    return f"{k:.2f} KB"