🪲 BUG-#69: Fix Background mode results unreliable

[FernandoCelmer]

Description

• dotflow/core/workflow.py: fixed Background._flow_callback to append task results to the queue (was pass)
• dotflow/core/workflow.py: fixed Background.get_tasks() to return self.queue instead of self.tasks
• dotflow/core/workflow.py: fixed Background.run() to execute tasks directly via _run_sequential() instead of delegating to Sequential (which used its own internal callback, bypassing Background's queue entirely)

Motivation and Context

Background mode was silently delegating execution to a Sequential instance, which used its own _flow_callback and never wrote results back to Background's queue. As a result, get_tasks() always returned the original unexecuted task list.

Closes #69

Types of changes

• Bug fix (change that fixes an issue)

Checklist

• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works