Support `_ipython_key_completions_` completions hook

Discussed in https://github.com/posit-dev/positron/discussions/8425

^{Originally posted by @astrowonk July 7, 2025}
iPython (and thus Jupyter) support the ability to expose custom autocomplete information in class definitions. Here is a silly one:

class DemoAutocomplete:

    """demo class"""

    def __getitem__(self, item):
        return item + item

    @staticmethod
    def _ipython_key_completions_():
        """required for custom autocompletion of subscriptables"""
        return list(range(10))

In iPython or Jupyter, autocomplete now works for the subscriptable instance of this class.

However in Positron console, the autocomplete suggestions are both irrelevant (appears to be every defined object) and don't include the key completions data:

It would be wonderful if we could have autocomplete parity with juptyer/ipython. I use custom classes for autocomplete extensively as we have many tables and other data structures whose names I can't remember, so I create helper subscriptable objects with defined _ipython_key_completions_ that allow me to access this data easily.

Possible Implementation

^{Originally posted by @seeM July 8, 2025}
It doesn't work currently because we don't use IPython's completions system, we go directly to a language server which can inspect the kernel's variables.

We'd probably implement this by patching Jedi as we've done before, or we could try to get it added in the Jedi repo. Here's the upstream Jedi issue that was tracking this but was closed at some point: https://github.com/davidhalter/jedi/issues/771

^{Originally posted by @astrowonk July 8, 2025}
The iPython docs claim that it will also look at dir for autocompletion suggestions, but the code for _get_keys does not appear to do anything with dir.

dir does not seem to help the Positron console either (so I guess Jedi also doesn't look at dir).

I was hoping I could just also define dir and Jedi/Positron would autocomplete my example but no such luck!

Start by tracing Positron's direct language-server completion path and compare it with IPython's _get_keys implementation in IPython/core/completer.py. Review the closed Jedi issue 771 and the existing Jedi patching approach mentioned in the discussion. Done means subscriptable objects exposing ipython_key_completions produce the expected key suggestions in the Positron console.