[Improvement] Redirect to existing email report/alert subscription when creating duplicate in Superset 6.1.0

[htcnokia]

Description

Currently, in Apache Superset 6.1.0, when a user attempts to subscribe to a dashboard (via Set up an email report) that they have already subscribed to, the system returns a generic "Failed to create report" (or creation error) notification.

This behavior provides a poor user experience (UX) because:

1. It leads the user to believe a system or network error occurred, rather than a duplicate entry conflict.
2. It requires the user to manually close the modal, navigate to the Alerts & Reports management page, and search for their existing subscription to make updates or view details.

Desired Behavior / Proposed Solution

Instead of failing with an error prompt, Superset should handle duplicate subscription attempts gracefully:

1. Detect Existing Subscription: When a user clicks to create a subscription for a dashboard they are already subscribed to, check if an active report/alert already exists for this user and resource.
2. Auto-Redirect / Open Existing Config: Automatically open the existing subscription configuration modal for that report (or display an informative notification with a direct link/redirect to the existing report in Manage Alerts & Reports).
3. Clear User Feedback: Display a helpful info toast (e.g., "You are already subscribed to this dashboard. Opening your existing subscription settings...") instead of an error message.

Screenshots / Mockups

(Add any screenshots of the current error modal vs. expected behavior if available)

Environment

• Superset version: 6.1.0
• Feature: Dashboard Email Reports / Alerts

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