Fixes bug whereby read_only bool is called as a function

[technige]

This is a simple bug fix to remedy the following issue:

  File ".../site-packages/prompt_toolkit/filters/app.py", line 374, in emacs_insert_mode
    or app.current_buffer.read_only()
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Exception 'bool' object is not callable

It would appear to be a simple typo, calling the boolean read_only property as if it were a function.

[jonathanslenders]

Hi @technige, how do you reproduce this? Buffer.read_only is supposed to be of type Filter which is a callable.

[technige]

@jonathanslenders I had code which, while within a keypress handler, attempted to disable and then re-enable a Buffer by using buffer.read_only = False followed later by buffer.read_only = True. The full stack trace was as follows:

Unhandled exception in event loop:
  File "/usr/lib/python3.12/asyncio/events.py", line 88, in _run
    self._context.run(self._callback, *self._args)
  File ".../site-packages/prompt_toolkit/input/vt100.py", line 162, in callback_wrapper
    callback()
  File ".../site-packages/prompt_toolkit/application/application.py", line 714, in read_from_input_in_context
    context.copy().run(read_from_input)
  File ".../site-packages/prompt_toolkit/application/application.py", line 694, in read_from_input
    self.key_processor.process_keys()
  File ".../site-packages/prompt_toolkit/key_binding/key_processor.py", line 273, in process_keys
    self._process_coroutine.send(key_press)
  File ".../site-packages/prompt_toolkit/key_binding/key_processor.py", line 171, in _process
    matches = self._get_matches(buffer)
              ^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../site-packages/prompt_toolkit/key_binding/key_processor.py", line 129, in _get_matches
    return [b for b in self._bindings.get_bindings_for_keys(keys) if b.filter()]
                                                                     ^^^^^^^^^^
  File ".../site-packages/prompt_toolkit/filters/base.py", line 134, in __call__
    return all(f() for f in self.filters)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../site-packages/prompt_toolkit/filters/base.py", line 134, in <genexpr>
    return all(f() for f in self.filters)
               ^^^
  File ".../site-packages/prompt_toolkit/filters/base.py", line 177, in __call__
    return any(f() for f in self.filters)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../site-packages/prompt_toolkit/filters/base.py", line 177, in <genexpr>
    return any(f() for f in self.filters)
               ^^^
  File ".../site-packages/prompt_toolkit/filters/base.py", line 254, in __call__
    return self.func()
           ^^^^^^^^^^^
  File ".../site-packages/prompt_toolkit/filters/app.py", line 374, in emacs_insert_mode
    or app.current_buffer.read_only()
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Exception 'bool' object is not callable

But based on your explanation, I see that this is actually user error, with me incorrectly assuming that I could use this property in this way. Apologies for the noise, please feel free to close the PR.