Clean up NotImplementedError stubs in async core module

[btoron]

Is your feature request related to a problem? Please describe.

The async core module (ofsc/async_client/core/_base.py) contains 5 methods that raise NotImplementedError with no context about when they'll be implemented or which issues track them:

Method	Line	Related Sync Method
search_activities()	~337	OFSC.core.search_activities()
move_activity()	~340	OFSC.core.move_activity()
bulk_update()	~343	OFSC.core.bulk_update()
(2 additional stubs)	~1450-1453	TBD

These stubs provide no indication of:

• Why they're not yet implemented
• Which issue tracks the implementation
• What workaround users should use

Describe the solution you'd like

For each stub, either:

1. Implement it if the sync version already exists and it's straightforward to port
2. Link to a tracking issue with a meaningful error message:

   async def search_activities(self, params: dict):
       """Search activities by criteria. See Issue #123 for async implementation."""
       raise NotImplementedError(
           "search_activities is not yet available in the async client. "
           "Track progress: https://github.com/btoron/pyOFSC/issues/123"
       )

3. Remove it if it's not planned (dead code)

Describe alternatives you've considered

• Leave as-is (confusing for users who discover these methods)
• Add @deprecated decorator pointing to tracking issue

Additional context

Related issues that may cover some of these:

• Async Core API: Activities (Custom Actions) #123 (Activities Custom Actions)
• Async Core API: Resources (Custom Actions) #119 (Resources Custom Actions)