Admin panel: HTTP status code inconsistency in suggestion processing endpoint

[coderabbitai]

Description

In the admin panel's suggestion processing endpoint (/admin/suggestions/<suggestion_id>), there is an HTTP status code inconsistency.

When a suggestion has already been processed (status != 'pending'), the endpoint returns a 400 (Bad Request) status code. However, the @spec.validate decorator declares HTTP_409=ErrorResponse, and semantically, a 409 (Conflict) status would be more appropriate for this situation.

Context

• File: goodmap/core_api.py
• Endpoint: PUT /admin/suggestions/<suggestion_id>
• Current behavior: Returns 400 for already-processed suggestions
• Expected behavior: Should return 409 to match the spec and semantic meaning

References

• PR feat: better model handling when suggesting new point and migrate API from RESTX to Spectree #309: feat: better model handling when suggesting new point and migrate API from RESTX to Spectree #309
• Original review comment: feat: better model handling when suggesting new point and migrate API from RESTX to Spectree #309 (comment)
• Related to admin panel tracking issue: Add authentication to admin endpoints and separate admin blueprint #310

Reported by: @raven-wing