feat(displayhook): add register_hook/unregister_hook to ZMQShellDisplayHook

[rgbkrk]

Summary

Mirrors the register_hook / unregister_hook / _hooks chain that ZMQDisplayPublisher already has, but on ZMQShellDisplayHook — the displayhook used for the bare-last-expression execute_result path. finish_displayhook now runs the chain before session.send, with the same contract as ZMQDisplayPublisher.publish:

• hook(msg) -> msg — pass through, continue the chain, eventually session.send.
• hook(msg) -> None — hook handled the send itself; suppress the default send.

The hook list lives on a threading.local() so registrations don't leak across threads, matching the existing implementation on ZMQDisplayPublisher.

Motivation

DisplayFormatter is the documented extension point for mime formatting, but inside ipykernel display_data and execute_result take different paths after formatting:

• ZMQDisplayPublisher.publish (used by display() / display_data) builds the message and walks self._hooks before calling session.send. That's where buffer-attaching transforms, redirectors, and similar extensions live today.
• ZMQShellDisplayHook.finish_displayhook (used for the bare-last-expression execute_result path) calls session.send directly with no hook chain.

So an extension that wants identical behavior on both paths — for example, attaching ZMQ buffers to the outbound message — has to subclass ZMQShellDisplayHook and reimplement finish_displayhook from scratch. This PR makes the two paths symmetric so a single hook function can register_hook on both seats.

Concrete use case

Streaming Arrow IPC bytes alongside a small JSON envelope on display_data / execute_result, without going through comms or widgets. The kernel emits a normal display message whose data carries a structured payload (mimetype + ref) and attaches the Arrow bytes as ZMQ buffers; the frontend reassembles. This pattern composes naturally with __arrow_c_stream__, so any dataframe library that implements the protocol gets a high-fidelity representation without per-library shims.

Doing this on display_data works today via ZMQDisplayPublisher.register_hook. Doing it on execute_result (the bare df on the last line of a cell) requires the same hook to fire on ZMQShellDisplayHook, which it can't today. nteract currently carries a subclass to bridge the gap; with this PR upstream, the subclass goes away.

Compatibility

• No behavior change when no hooks are registered: the existing guards (self.msg, content.data non-empty, self.session configured) are preserved, and session.send is called exactly as before.
• The hook contract is identical to ZMQDisplayPublisher's, so a hook authored against display_data works unchanged on execute_result.
• Thread-local storage means existing single-threaded extensions see no difference; multi-threaded callers get the same isolation ZMQDisplayPublisher already provides.