fix: error_message cannot be cleared on task status update

[bsbodden]

Problem

update_task_status in agent_memory_server/tasks.py only sets error_message when the provided value is not None:

if error_message is not None:
    task.error_message = error_message

This means once an error_message is set on a task, it can never be cleared. If a task transitions from FAILED back to a new attempt (or if the error message needs to be reset), the stale error persists.

Impact

A task that previously failed and retains its error_message even after a successful resolution creates confusing state: status=SUCCESS + error_message="Connection refused".

Proposed Fix

Use a sentinel value pattern to distinguish "caller didn't pass error_message" from "caller wants to clear error_message". This allows error_message="" or a dedicated _UNSET sentinel to clear the field.

Related

Part of #190 (Summary Views and Tasks)