fix: flash_backup validation, smoke test assertions, docker/notification test coverage

- storage.py: validate initiateFlashBackup response before returning success=True
- test-tools.sh: remove set -e/inherit_errexit; add success assertion to smoke tests
- test_destructive_guards.py: add confirm-guard tests for new docker destructive actions
- test_docker.py: assert mutation variables in organizer tests; add items branch test
- test_query_validation.py: add 5 missing notification mutation schema test methods
- test_notifications.py: use lowercase importance to test uppercasing logic

Resolves review threads PRRT_kwDOO6Hdxs50FgPb PRRT_kwDOO6Hdxs50FgO4 PRRT_kwDOO6Hdxs50FgO8 PRRT_kwDOO6Hdxs50FgPI PRRT_kwDOO6Hdxs50FgPL PRRT_kwDOO6Hdxs50FgPm PRRT_kwDOO6Hdxs50E2iK PRRT_kwDOO6Hdxs50E2im

unraid_mcp/tools/storage.py

@@ -157,10 +157,13 @@ def register_storage_tool(mcp: FastMCP) -> None:
             with tool_error_handler("storage", action, logger):
                 logger.info("Executing unraid_storage action=flash_backup")
                 data = await make_graphql_request(MUTATIONS["flash_backup"], {"input": input_data})
+                backup = data.get("initiateFlashBackup")
+                if not backup:
+                    raise ToolError("Failed to start flash backup: no confirmation from server")
                 return {
                     "success": True,
                     "action": "flash_backup",
-                    "data": data.get("initiateFlashBackup"),
+                    "data": backup,
                 }
 
         query = QUERIES[action]