Multiple extensions using `get_base_class_hook` interfere with each other

Bug Report

The first plugin that uses get_base_class_hook and returns a callback appears to take exclusive ownership of that hook, preventing other plugins from intercepting the same class definitions.

I specifically noticed this with the mypy_zope plugin which uses get_base_class_hook to analyze every class. And then writing my own custom mypy plugin where I also want to use get_base_class_hook but it wasn't getting called at all.

To Reproduce

1. Setup two plugins using get_base_class_hook. To notice the effect, they should return a callback instead of None.
   mypy_plugin1.py

   from typing import Callable, Optional, Type
   
   from mypy.plugin import (
       ClassDefContext,
       Plugin,
   )
   
   
   class Plugin1(Plugin):
       def get_base_class_hook(
           self, fullname: str
       ) -> Optional[Callable[[ClassDefContext], None]]:
           def _analyze_class(classdef_ctx: ClassDefContext) -> None:
               print("plugin1 get_base_class_hook", fullname)
   
           return _analyze_class
   
   
   def plugin(version: str) -> Type[Plugin1]:
       return Plugin1
   

   mypy_plugin2.py

   from typing import Callable, Optional, Type
   
   from mypy.plugin import (
       ClassDefContext,
       Plugin,
   )
   
   
   class Plugin2(Plugin):
       def get_base_class_hook(
           self, fullname: str
       ) -> Optional[Callable[[ClassDefContext], None]]:
           def _analyze_class(classdef_ctx: ClassDefContext) -> None:
               print("plugin2 get_base_class_hook", fullname)
   
           return _analyze_class
   
   
   def plugin(version: str) -> Type[Plugin2]:
       return Plugin2
   

2. Configure mypy to use those plugins. Edit mypy.ini and add plugins = scripts-dev/mypy_plugin1.py, scripts-dev/mypy_plugin2.py
3. Lint some files, poetry run mypy
4. Notice that there is only logs from plugin1

Expected Behavior

mypy calls get_base_class_hook in every plugin (logs for plugin1 and plugin2 show up)

Actual Behavior

plugin1 blocks other plugins from hooking into get_base_class_hook (only logs from plugin1 show up).

Your Environment

• Mypy version used: v1.16.1
• Mypy command-line flags: (none)
• Mypy configuration options from mypy.ini (and other config files): plugins = scripts-dev/mypy_plugin1.py, scripts-dev/mypy_plugin2.py (real-life use case)
• Python version used: 3.13.3

Inizia riproducendo il comportamento con mypy_plugin1.py e mypy_plugin2.py, configurati tramite mypy.ini, ed esegui poetry run mypy. Leggi la documentazione di get_base_class_hook e segui come vengono interrogati i plugin configurati; è completato quando vengono invocati i callback di entrambi i plugin per le stesse definizioni di classe.