insert local config file path to config file page

[nathangavin]

Description

Fixes #21478

Admin > Diagnostic > Config file page currently has a hardcoded value for the config file path. This change instead uses the local path of the instance config file.

Checklist

• [✔] I have understood, reviewed, and tested all AI outputs before use
• [✔] All AI instructions respect security, IP, and privacy rules

Review

• Functional review done
• Potential edge cases thought about (behaviour of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• New documentation added or existing documentation updated

[sgiehl]

A UI test is failing. Looks a bit strange, that the path now starts with //. Might be good to check the reason instead of simply updating the screenshot.

[nathangavin]

A UI test is failing. Looks a bit strange, that the path now starts with //. Might be good to check the reason instead of simply updating the screenshot.

Looks like the PIWIK_USER_PATH is being set to var/www/html/ (test/PHPUnit/proxy/includes.php), so when the config file path is generated, you get a double slash var/www/html//config/config.ini.php. Somewhere in the tests that first section is removed from paths in UI tests, so you end up with //config/config.ini.php. I've updated the logic such that PIWIK_USER_PATH is set to var/www/html in tests, so we end up with a path with the correct slashes. The only side-effect of this change is the fact that PIWIK_USER_PATH is no longer considered custom, so the Diagnostic UI screenshot needed updating too.