`user_data.clear()` orphans running auth polling tasks

[Vachhani-Tapan]

🐛 Bug Description

Both cancel_commit_callback and cancel_handler call context.user_data.clear(), which removes ALL keys including the running auth_task reference. The background _poll_device_auth task continues running as an orphan with no way to track or cancel it.

🔁 Steps to Reproduce

1. Start the /auth GitHub Device Flow.
2. Cancel the flow via cancel button before it completes.
3. Observe that the background polling task continues running.

✅ Expected Behavior

Cancelling the auth flow should properly cancel the background polling task.

❌ Actual Behavior

The orphaned polling task may still send success/failure notifications to the user long after cancellation. The task reference is lost so it cannot be properly managed.

🌍 Environment

Field	Value
OS	Any
VS Code version	Any
GitPhone extension version	Any
Python version	3.11+

📋 Additional Context

File: backend/bot.py:1228, 1234-1239, 1241
Fix: Cancel the auth task explicitly before clearing user data.

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[Vachhani-Tapan]

@ankittroy-21 /claim Assign me this issue under ECSoC'26

[karrisanthoshigayatri]

/claim