[FEATURE] HookRegistry.add_callback should return a cleanup function for unregistering

[opieter-aws]

Problem Statement

add_callback() in registry.py returns None. Once a callback is registered, there is no mechanism to unregister it. TypeScript's addCallback() returns a HookCleanup function that removes the callback when called.

Use Case

Dynamic hook management during agent lifetime:

• Temporarily adding a logging hook for debugging, then removing it
• Plugins that register hooks on init and clean up on teardown
• Test fixtures that add hooks and need to clean up

Proposed Solution

Return a cleanup callable from add_callback():

def add_callback(self, event_type, callback):
    self._callbacks[event_type].append(callback)
    def cleanup():
        self._callbacks[event_type].remove(callback)
    return cleanup

Additional context

This was a finding from an agentic cross-SDK consistency scan between Python and TypeScript SDKs (strands-agents/sdk-typescript#789). The finding has been triaged, but verify that this needs an actual fix.

[notowen333]

From a UX perspective, lets see if there's a path to cleaning these up in a reasonable way by default. And then also offering the ability for devs to manage the lifecycle fully