fix(parser): keep a mode's directly-assigned allowedModes when it also declares categories

setModeRegistry() replaced $this->allowedModes with the category-derived list
whenever a mode declared cate

fix(parser): keep a mode's directly-assigned allowedModes when it also declares categories

setModeRegistry() replaced $this->allowedModes with the category-derived list
whenever a mode declared categories, discarding any mode names the subclass had
assigned to the property directly. The old SyntaxPlugin::accepts() merged the
category modes into the existing list instead, so a mode using the
sibling-component pattern - a non-empty getAllowedTypes() plus
$this->allowedModes[] = 'plugin_foo_bar' in its constructor - kept both and its
sibling syntax nested as intended.

Merge the category-derived modes into the directly-assigned list rather than
replacing it, then deduplicate. The no-categories path is unchanged: the
directly-assigned list is used as-is.

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