fix: align proxy latency testing with Mihomo client behavior

[Felyx-Fu]

Problem

MioProxy currently reports significantly higher latency than other Mihomo-based clients for the same subscription and nodes.

Example:

• Other Mihomo clients: ~50–200 ms
• MioProxy: several hundred ms, sometimes close to 1000 ms

Current behavior

MioProxy calls Mihomo:

/proxies/{proxy}/delay

without passing the proxy group's configured test URL.

The backend then falls back to:

https://www.gstatic.com/generate_204

with a 5000 ms timeout.

Reference behavior

Clash Party uses the following priority:

1. User-configured delay test URL
2. Proxy group's own url
3. https://www.gstatic.com/generate_204

It also uses provider health checks where appropriate.

Expected behavior

MioProxy should:

• respect a proxy group's configured test URL;
• support a configurable default latency test URL;
• support configurable timeout;
• keep HTTPS as the safe default;
• avoid modifying latency values artificially;
• preserve direct use of Mihomo's delay API.

Acceptance criteria

• Same node produces broadly comparable results to other Mihomo clients under equivalent test conditions.
• Group-specific test URLs are respected.
• Default behavior remains secure and HTTPS-based.
• Existing proxy selection and profile behavior is unaffected.