Custom MCP connector removed without warning and cannot be re-added

### What happened?

A custom MCP connector I had configured and was actively using (a Supabase-backed server) was removed from my account without any action on my part. Mid-session, all of its tools became unreachable — calls that had been working moments earlier began returning "Tool not found" errors. When I went to re-add the connector, I was unable to: the custom connector could not be re-created/re-authenticated. Associated custom skills tied to this setup were also no longer available. Image upload in the same session was also failing, though that may be unrelated.

### What did you expect to happen?

The custom MCP connector should have remained connected and usable for the duration of my session. If it needed to be removed or re-authenticated, I expected a clear notification and a working path to re-add it. I expected to be able to re-create the custom connector from the connector management UI without error.

### Steps to reproduce

Configure and connect a custom MCP server (Supabase-backed connector) and use its tools successfully in a conversation.
Continue the session; observe that the connector's tools stop responding mid-session — tool calls return "Tool not found."
Open the connector management UI and attempt to re-add / re-authenticate the custom connector.
Observe that the connector cannot be re-added.

### Area

MCP Connector (adding/managing servers)

### MCP Server (if applicable)

Custom Supabase-backed MCP server

### Error messages or logs

```shell
Tool '' not found
(Occurred on previously-working tool calls after the connector was removed mid-session. Replace with the specific tool names if you have them — e.g. the query/update tools you were calling.)
```

### Additional context

The connector was working normally earlier in the same session, then became unavailable without any user action. Custom skills associated with this configuration also disappeared at the same time, and image upload was failing in the same session — flagging in case these are related to the same underlying incident. This interrupted active work. Please advise whether the connector and skills can be restored, or how to successfully re-add a custom connector.

No repository files or tests are named. Start by reviewing the MCP connector management UI and reproducing the mid-session removal, the "Tool not found" response, and the failed re-add or re-authentication for the custom Supabase-backed server. Done means the connector remains usable, removal is reported clearly, and a removed connector can be re-added successfully.

Scritto dal modello di indicizzazione a partire dal testo della issue.