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

Find where the settings UI’s Test upstreams button sends its request, then trace that API path in the backend validation code. Inspect how upstream host/port entries are parsed and how the response maps to green OK versus red error feedback, then add a direct self-reference check for loopback endpoints and local listening ports. Run the relevant upstream-validation/endpoint tests and confirm localhost/127.0.0.1:53 now returns an error state.

Escrito por el modelo de indexación a partir del texto del issue.