Implement todos for session failures

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Problem

Currently, when an agent session fails, users have no notification mechanism to alert them of the failure. This means users must actively monitor the sessions detail page to discover when a session has failed, which can lead to:

• Delayed awareness of session failures or successes
• Wasted time waiting for sessions that have already failed
• Poor user experience when working with agents and flows
• Missed opportunities to quickly address and retry failed sessions

Unlike pipelines, which generate todos on failure, sessions lack this established notification pattern, leaving users without a reliable way to be informed of terminal failure states.

Desired Outcome

Implement a todo notification system for session failures that follows GitLab's established patterns:

• When a session fails, automatically generate a todo for the user who initiated the session
• Use copy and styling from #580834