refactor(guards): migrate array/keys/plugins to gate_destructive_action

Replace 7-11 line inline guard blocks in array.py, keys.py, and plugins.py
with single await gate_destructive_action(...) calls. Also fix guards.py to
raise unraid_mcp.core.exceptions.ToolError (project subclass) instead of
fastmcp.exceptions.ToolError so pytest.raises catches it correctly in tests.

unraid_mcp/core/guards.py

@@ -10,9 +10,8 @@ from typing import TYPE_CHECKING
 if TYPE_CHECKING:
     from fastmcp import Context
 
-from fastmcp.exceptions import ToolError
-
 from ..config.logging import logger
+from .exceptions import ToolError
 
 
 async def elicit_destructive_confirmation(