* fix include
* return back deleted file
* Fix maint include guard: test file_exists on path, not require_once return
file_exists() was wrapping require_once(), so the first require ran unconditionally and its integer return (1) was passed to file_exists(), which fell through to require the non-existent includes/functions.php and fatalled. Test the path strings directly.
* Cover maint include fallback and error branches
The maint-include lookup in thold_check_threshold() and thold_update_host_status() gained an includes/functions.php fallback and an error-log branch, but the suite only exercised the plugin-root path. Add MaintIncludeResolutionTest driving all three outcomes for both functions, with cacti-root fixtures for the includes/ layout and the missing-plugin case, so patch coverage of the changed lines reaches 100%.
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Co-authored-by: TheWitness <thewitness@cacti.net>
