Cannot add two custom MCP connectors with the same server URL (blocks multi-account use cases)

### What happened?

Claude.ai rejects adding a second custom MCP connector if the Remote MCP server URL is already used by another connector, with the error:

Error: A server with this URL already exists

This blocks any legitimate multi-account use case where the same MCP server (e.g. Stripe, Notion, Linear) needs to be connected with different OAuth identities.

### What did you expect to happen?

Allow multiple connectors with the same MCP server URL as long as the connector **name** is unique. Each connector would maintain its own independent OAuth session and credentials. The combination of (URL, name) should be the uniqueness key, not URL alone.

This is especially important for Claude Code routines, where per-routine connector selection is already supported but loses meaning if you can only have one Stripe (or one Notion, etc.) connector per account.

### Steps to reproduce

1. Add a custom connector named `Stripe Account A` with URL `https://mcp.stripe.com` → authenticate via OAuth with Stripe account A ✅
2. Try to add a second custom connector named `Stripe Account B` with the same URL `https://mcp.stripe.com` → ❌ blocked with "A server with this URL already exists"

### Area

MCP Connector (adding/managing servers)

### MCP Server (if applicable)

_No response_

### Error messages or logs

```shell
Error: A server with this URL already exists
```

### Additional context

_No response_