[Server] Fix conformance sampling test stuck issue with Docker PHP-FPM

[CodeWithKyrian]

This PR introduces a Docker-based conformance testing infrastructure using Nginx and PHP-FPM, enabling proper testing of sampling functionality.

Motivation & Context

The sampling tool was stuck during conformance tests (#211). As explained in my comment on the issue, this happens because PHP's built-in server (php -S) is single-threaded.
I chose Nginx + PHP-FPM over alternatives (Symfony CLI, FrankenPHP) because it's the most production-realistic setup and tests what users will actually deploy.

What's Changed

• Added Docker-based conformance server with Nginx + PHP-FPM
• Moved conformance tests from examples/server/conformance/ to tests/Conformance/
• Updated Makefile with a single conformance-tests target
• Updated CI pipeline to use Docker-based conformance testing
• Enabled the test_sampling tool in conformance server

Fixes #211

[chr-hertel]

Hey @CodeWithKyrian, thanks for jumping on that - I guess that's the way to go 👍

However, just checking out the branch and running make conformance-tests doesn't work right away - will have another look later, but assume it's working for you looking at the pipeline 👍

okay, was an issue with logs and sessions folder being not writable to the php runtime within the container - could be a local issue tho

[chr-hertel]

Thanks @CodeWithKyrian! 👍