prometheus: Settings: Fix connection test crash on plain-text response

[Athang69]

Summary

This PR fixes a crash in the Prometheus plugin connection test by replacing the plain text /-/healthy health check with a JSON producing query endpoint.

Related Issue

Fixes kubernetes-sigs/headlamp#6541

Changes

• Updated the connection test in Settings.tsx to query /api/v1/query?query=up instead of /-/healthy
• Added a status check on the response body to confirm the query API actually succeeded, not just that something answered on the port

Steps to Test

1. Deploy a standard external Prometheus server and expose it as a service in your cluster, for example on port 9090
2. In Headlamp, open the Prometheus plugin settings and configure the namespace, service, and port pointing to that Prometheus instance
3. Click Test Connection and confirm it succeeds instead of throwing a JSON parse error in the console

Notes for the Reviewer

• Standard Prometheus servers return text/plain from /-/healthy, for example "Prometheus Server is Healthy.", which crashed the JSON parser with "Unexpected token 'P'... is not valid JSON"
• Swapped to /api/v1/query?query=up, which always returns JSON and validates the query engine end to end rather than just liveness
• Verified locally with a script that mocks both the old and new proxy responses, reproducing the original crash and confirming the fix avoids it, without needing a live cluster
• Also confirmed clean tsc, eslint, and npm run build on this branch

[Copilot]

Pull request overview

This PR updates the Prometheus plugin Settings “Test Connection” flow to avoid JSON parsing crashes by switching from the plain-text /-/healthy endpoint to the JSON-based Prometheus query API, and it adds a basic validation of the Prometheus API response.

Changes:

• Switch connection test endpoint from /-/healthy to /api/v1/query?query=up (JSON response).
• Validate the returned Prometheus payload by checking response.status === 'success' before reporting success.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.