Failed OAuth leaves custom connector registered but hidden — can't authenticate, remove, or re-add ("URL already exists")

### What happened?

Adding a custom connector (remote MCP) and then not completing the OAuth sign-in leaves the connector registered on the account but invisible in the UI, with no way to recover it.

Concretely:
- The connector does NOT appear in Settings → Connectors, so there is no entry to authenticate or to remove.
- Re-adding the SAME URL is rejected with: "A server with this URL already exists."
- Net result: it can't be connected, can't be removed, and can't be re-created. The URL is effectively locked for the account.

## Steps to reproduce
1. Settings → Connectors → Add custom connector
2. Enter a Name and a remote MCP server URL that uses OAuth (e.g. `https://services.leadconnectorhq.com/mcp/anthropic/v2`)
3. Leave OAuth Client ID/Secret blank and click Add
4. The OAuth sign-in popup opens
5. Close/cancel the popup without finishing sign-in
6. Dialog closes; no connector appears in the Connectors list
7. Reopen "Add custom connector", enter the SAME URL, click Add
8. Error: "A server with this URL already exists"

## Expected behavior
An unauthenticated/incomplete connector should either:
- appear in the list in a "needs authentication / Connect" state so it can be completed or removed, OR
- have its URL released (since auth was never completed) so it can be re-added.

## Actual behavior
The connector is hidden from the UI but still reserved server-side, leaving no path to authenticate, remove, or re-add it.

## Environment
- Claude.ai (web) — Max plan
- Browser: [fyll i, t.ex. Chrome 1XX / macOS]
- Connector type: custom remote MCP (OAuth)

### What did you expect to happen?

I aspected to be able to try again

### Steps to reproduce

I Expected to be able to try again

### Area

MCP Connector (adding/managing servers)

### MCP Server (if applicable)

_No response_

### Error messages or logs

```shell

```

### Additional context

_No response_

Start by reproducing the flow in Settings → Connectors: add a custom remote MCP server using OAuth, cancel the sign-in, then try to re-add the same URL. Trace connector creation, OAuth cancellation, visibility in the connector list, and duplicate-URL validation. Done means an incomplete connector can be authenticated or removed, or its URL can be added again.

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