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

tests/http_layer/test_request_construction.py

@@ -19,6 +19,7 @@ from tests.conftest import make_tool_fn
 from unraid_mcp.core.client import DEFAULT_TIMEOUT, DISK_TIMEOUT, make_graphql_request
 from unraid_mcp.core.exceptions import ToolError
 
+
 # ---------------------------------------------------------------------------
 # Shared fixtures
 # ---------------------------------------------------------------------------
@@ -582,7 +583,7 @@ class TestVMToolRequests:
             return_value=_graphql_response({"vm": {"stop": True}})
         )
         tool = self._get_tool()
-        result = await tool(action="stop", vm_id="vm-456")
+        await tool(action="stop", vm_id="vm-456")
         body = _extract_request_body(route.calls.last.request)
         assert "StopVM" in body["query"]
         assert body["variables"] == {"id": "vm-456"}