`PluginRegistry` persists unregistered plugins

def test_theme_unregister() -> None:
    @theme.register("big square", enable=True)
    def custom_theme() -> ThemeConfig:
        return {"height": 1000, "width": 1000}

    assert theme.active == "big square"
    fn = theme.unregister("big square")
    assert fn is not None
    assert fn() == custom_theme()
    assert theme.active == theme.themes.active
    assert theme.active != "big square"
FAILED tests/vegalite/v5/test_theme.py::test_theme_unregister - AssertionError: assert 'big square' != 'big square'

import altair as alt

alt.themes.active
# 'default'

def theme_1() -> Any:
    return {"height": 1000, "width": 1000}


alt.themes.register("theme 1", theme_1)
alt.themes.register("theme 2", lambda: {"height": 500, "width": 500})

{"theme 1", "theme 2"} < set(alt.themes.names())
# True

alt.themes.enable("theme 1")

alt.themes.active
# 'theme 1'

unregistered = alt.themes.register("theme 1", None)
unregistered is not None
# True

unregistered()
# {'height': 1000, 'width': 1000}

alt.themes.active
# 'theme 1'

"theme 1" not in alt.themes.names()
# True

alt.Chart("data.csv").mark_bar().encode(x="x:Q").to_dict()
{'height': 1000,
 'width': 1000,
 'data': {'url': 'data.csv'},
 'mark': {'type': 'bar'},
 'encoding': {'x': {'field': 'x', 'type': 'quantitative'}},
 '$schema': 'https://vega.github.io/schema/vega-lite/v5.20.1.json'}

active_theme = alt.themes.get()

active_theme() == unregistered() 
# True

alt.themes.enable()

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
File c:/Users/danie/Documents/GitHub/altair/altair/utils/plugin_registry.py:198, in PluginRegistry._enable(self, name, **options)
    197 try:
--> 198     (ep,) = (
    199         ep
    200         for ep in importlib_metadata_get(self.entry_point_group)
    201         if ep.name == name
    202     )
    203 except ValueError as err:

ValueError: not enough values to unpack (expected 1, got 0)

The above exception was the direct cause of the following exception:

NoSuchEntryPoint                          Traceback (most recent call last)
Cell In[21], line 5
      3 with warnings.catch_warnings():
      4     warnings.filterwarnings("ignore", category=alt.AltairDeprecationWarning)
----> 5     alt.themes.enable()

File c:/Users/danie/Documents/GitHub/altair/altair/vegalite/v5/theme.py:61, in ThemeRegistry.enable(self, name, **options)
     33 def enable(
     34     self,
     35     name: LiteralString | AltairThemes | VegaThemes | None = None,
     36     **options: Any,
     37 ) -> PluginEnabler[Plugin[ThemeConfig], ThemeConfig]:
     38     """
     39     Enable a theme by name.
     40 
   (...)
     59     Default `vega` themes can be previewed at https://vega.github.io/vega-themes/
     60     """
---> 61     return super().enable(name, **options)

File c:/Users/danie/Documents/GitHub/altair/altair/utils/plugin_registry.py:240, in PluginRegistry.enable(self, name, **options)
    238 if name is None:
    239     name = self.active
--> 240 return PluginEnabler(self, name, **options)

File c:/Users/danie/Documents/GitHub/altair/altair/utils/plugin_registry.py:61, in PluginEnabler.__init__(self, registry, name, **options)
     59 self.options: dict[str, Any] = options
     60 self.original_state: dict[str, Any] = registry._get_state()
---> 61 self.registry._enable(name, **options)

File c:/Users/danie/Documents/GitHub/altair/altair/utils/plugin_registry.py:207, in PluginRegistry._enable(self, name, **options)
    205         raise ValueError(self.entrypoint_err_messages[name]) from err
    206     else:
--> 207         raise NoSuchEntryPoint(self.entry_point_group, name) from err
    208 value = cast(PluginT, ep.load())
    209 self.register(name, value)

NoSuchEntryPoint: No 'theme 1' entry point found in group 'altair.vegalite.v5.theme'