scripts: fix crash when loading addon scripts that define a dataclass - #8293

Description

Loading an addon script that defines a @dataclass currently crashes:

@dataclass
  File ".../dataclasses.py", line ..., in _process_class
    ... = sys.modules.get(cls.__module__).__dict__
AttributeError: 'NoneType' object has no attribute '__dict__'

The root cause is in mitmproxy.addons.script.load_script: it builds the
module with importlib.util.module_from_spec(spec) and then calls
loader.exec_module(m) without first registering the module in
sys.modules. Python's import documentation recommends inserting the module
into sys.modules before executing it, because parts of the standard library
(dataclasses, pickling, typing.get_type_hints, ...) look the defining
module up in sys.modules by its __module__ name while the class body runs.
When the entry is missing the lookup returns None and the decorator raises.

This registers the module in sys.modules before execution and removes it
again if execution fails, so a failed load still leaves nothing behind. The
existing sys.modules.pop(fullname, None) at the top of load_script keeps
guaranteeing uniqueness across reloads.

Closes #8279

Checklist

• I have updated tests where applicable.
• I have added an entry to the CHANGELOG.

Summary

• Register addon-script modules in sys.modules before exec_module, and
  unregister on failure.
• Add a regression test (test_load_script_with_dataclass) and a
  dataclass.py addon fixture that fails to load without the fix.
• Add a CHANGELOG entry.