Fix UniFi app credential test validation

[jaycollett]

Summary

Fixes the UniFi app test() method which was incorrectly reporting "Invalid credentials" even when credentials were valid and the dashboard stats were working correctly.

Root Cause

The original test() method used appTest() which only checked HTTP status codes. This approach had several issues:

1. Self-hosted controllers return HTTP 400 (not 401) on authentication failure, with error details in the response body
2. UDM devices might return HTTP 200 but still have authentication issues that need body validation
3. The test didn't verify the session actually worked (unlike livestats() which implicitly validates by successfully fetching data)

Changes

• Use execute() directly with explicit POST method (matching livestats() behavior)
• Handle HTTP 400 errors for self-hosted controller authentication failures
• Handle HTTP 401/403 errors for UDM authentication failures
• Validate self-hosted response body (meta.rc === "ok")
• Verify login success by attempting to fetch stats (same validation as dashboard)
• Provide descriptive error messages for different failure scenarios

Related Issues

• Fixes [BUG] - UniFI app *does* show statistics; but testing credentials fails #811 - UniFi app shows statistics but testing credentials fails (UDM)
• Should fix UniFI app does not show any statistics; test button shows invalid credentials #695 - UniFi app does not show any statistics; test button shows invalid credentials
• Should fix Unifi - Failed: Invalid credentials #640 - UniFi - Failed: Invalid credentials (self-hosted Docker)

Testing

Tested on:

• Device: UniFi Dream Machine Pro (UDM-Pro)
• UniFi OS Console: Latest version
• Network Application: Latest version
• Heimdall: 2.6.3 running in Docker

Test Results

Scenario	Before	After
Valid credentials	"Invalid credentials" error	✅ "Successfully connected to UniFi"
Invalid credentials	"Invalid credentials"	✅ "Failed: Invalid credentials"
Connection error	Generic error	✅ "Failed: Connection error"

Test Configuration

• "Self Hosted" toggle: unchecked (UDM-Pro uses built-in controller)
• URL: https:// endpoint (HTTP redirects to HTTPS on UDM)
• Skip TLS verification: Not required when using valid SSL certificate

Notes

• This fix was only tested on UDM-Pro hardware. Self-hosted controller testing would be beneficial to confirm fixes for Unifi - Failed: Invalid credentials #640 and UniFI app does not show any statistics; test button shows invalid credentials #695
• The fix maintains backward compatibility with existing configurations
• No changes to livestats() were needed as it was already working correctly

[mvdkleijn]

Sorry for the delay. I was busy working on https://opencommit.eu related stuff. Merging.