[Task] (heureka): Revert change severity remediation and update vulnerability status

[hodanoori]

Acceptance Criteria

• A Revert Change Severity action is available for changed severity vulnerabilities.
• Selecting the action successfully revokes the existing changed severity mitigation.
• After revert, the vulnerability status is updated correctly back to Active.
• The updated status is reflected consistently across all relevant tabs (e.g., Active and Vulnerabilities).
• UI updates immediately after the revert without requiring a page reload.