detect self-reference in "Test upstreams" button

### Prerequisites

- [x] I have checked the [Wiki](https://github.com/AdguardTeam/AdGuardHome/wiki) and [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions) and found no answer

- [x] I have searched other issues and found no duplicates

- [x] I want to request a feature or enhancement and not ask a question

### The problem

When configuring upstreams, there is a handy, blue **Test upstreams** button, but it does not detect errors such as these

```
localhost
localhost:53
127.0.0.1
127.0.0.1:53
[/local/]udp://127.0.0.1:53
```

My system had an entry like the last line with port 54, but when I accidently changed 54 to 53, it took a few minutes to troubleshoot, so it would be nice if AGH detected this in the existing feature to test upstreams.

### Proposed solution

**Test upstreams** detects a direct self-reference. Possible options

1. Active: send a specially crafted probe DNS request
2. Passive: parse the entry and compare to AGH configuration

Then, return a red error notification to the user (instead of the green OK notification).

### Alternatives considered and additional information

This issue, which is about a loop caused by direct self-reference, is distinct from https://github.com/AdguardTeam/AdGuardHome/issues/6693 which looks like an indirect circular loop. The direct reference seems easier to solve