[OHE] Improve MCP setup feedback when a server cannot connect

Problem

When a user adds an MCP server in the UI, the UI does not clearly show whether it connected successfully. If setup fails, the server's tools may simply be missing later, leaving the user unsure what happened or how to fix it.

Why this feature: This feature is still useful, even as we have OHE users make the transition to Agent Canvas. There will be an overlap period where OHE users may still use the 'legacy' UI, and this is a small quality-of-life improvement for customers with dozens of users onboarding to OHE.

Desired experience

• Show a clear success or failure state after adding/editing a server, perhaps with a test connection button.
• Explain failures in plain language, such as connection, timeout, or authentication problems.
• Warn in the conversation when a configured server is unavailable.
• Never expose secrets or sensitive configuration details.

Acceptance criteria

Verify users can see a "Test Connection" option in both the OHE UI and Agent Canvas UI when connected to a Cloud backend.

This issue was updated by an AI agent (OpenHands) on behalf of a user.

Start by tracing the server add/edit flow in the OHE UI and Agent Canvas UI against the Cloud backend, focusing on where a Test Connection entry point can be exposed. Done means both UIs offer Test Connection and users receive clear, safe success or failure feedback without exposing secrets.