1. Built-in plugins (plugin_init):
A non-ASCII builtin name is a developer error. Added a check
using my_string_repertoire() that prints an error via
print_init_failed_error() and returns 1 to abort server startup.
2. Dynamically loaded plugins (plugin_add):
Added a pre-validation loop before the registration loop. If any
plugin name in the .so is non-ASCII, the entire .so is rejected
before any plugin is registered, preventing partial state where
valid plugins from the same .so get left INACTIVE in the hash.
Added test plugins and MTR tests to verify both rejection of
non-ASCII named plugins and successful loading of ASCII named plugins.
