fix(tags): disallow tag recommendations for restricted incidents

[whitdog47]

This PR updates the tag recommendation flow to disallow recommendations for restricted incidents. Key changes include:

• Raising a ValueError when the incident is not found.
• Returning an error message when the incident is restricted.

[Copilot]

Pull Request Overview

This PR updates the tag recommendation flow to disallow recommendations for restricted incidents. Key changes include:

• Raising a ValueError when the incident is not found.
• Returning an empty recommendation list with an error message when the incident is restricted.

Comments suppressed due to low confidence (1)

src/dispatch/ai/service.py:448

• [nitpick] The current error handling uses an exception for a missing incident and a response with an error message for a restricted incident. For consistency, consider aligning these approaches by either returning an error message in both cases or raising exceptions in both scenarios.

            return TagRecommendationResponse(recommendations=[], error_message=message)