fix: restore unreachable no-default-class error in load_plugin

[manunicholasjacob]

What this changes

load_plugin() reads generator_mod.DEFAULT_CLASS directly when a plugin is
specified as category.module. If the module does not define that attribute,
the attribute access raises AttributeError, so the else branch below it
cannot be reached:

if generator_mod.DEFAULT_CLASS:
    plugin_class_name = generator_mod.DEFAULT_CLASS
else:
    raise ValueError(
        f"module {module_name} has no default class; pass module.ClassName to target_type"
    )

Two consequences:

1. The ValueError above, and its guidance to pass module.ClassName, never
   surfaces. Users see an AttributeError instead.
2. The enclosing try only catches ValueError, so callers passing
   break_on_fail=False receive an exception rather than False.

This switches the read to a defaulted lookup so the existing error path works
as written. No behaviour changes for modules that do define DEFAULT_CLASS.

Reproduction

>>> _plugins.load_plugin("detectors.always", break_on_fail=False)
AttributeError: module 'garak.detectors.always' has no attribute 'DEFAULT_CLASS'

Also reachable from the CLI, e.g. --target_type base.

Testing

Adds test_load_plugin_module_without_default_class to
tests/plugins/test_plugin_load.py, covering both the break_on_fail=False
return value and the ValueError message. The test fails on main and passes
with this change. Full tests/plugins/ suite passes.