fix: remove unreachable created_at backfill in update_task_status

[bsbodden]

Problem

update_task_status in agent_memory_server/tasks.py contains dead code at lines 96-97:

# Ensure created_at is always set
if task.created_at is None:
    task.created_at = datetime.now(UTC)

This code is unreachable because Task.created_at is defined as datetime (non-optional) with default_factory=lambda: datetime.now(UTC). If a task somehow has created_at: null in Redis, Task.model_validate_json() raises a ValidationError before this backfill code can execute, making the task permanently unrecoverable.

Root Cause

The Task model field created_at: datetime does not accept None. The Pydantic validator rejects null values before the Python code has a chance to backfill.

Proposed Fix

1. Remove the dead backfill code from update_task_status
2. Make created_at explicitly Optional[datetime] with a default_factory, OR keep it non-optional (preferred — created_at should always exist on valid tasks)

Since create_task always produces tasks with created_at set (via default_factory), the simplest fix is to remove the dead code.

Related

Part of #190 (Summary Views and Tasks)