Testo: remove PHPUnit, finalize migration

[igancev]

Description

Remove PHPUnit from the project completely. Clean up all PHPUnit-related configuration, dependencies, and references.

What to do

• Remove phpunit/phpunit from require-dev in composer.json
• Delete phpunit.xml
• Remove any PHPUnit-related Makefile targets
• Remove any PHPUnit-related CI steps/jobs (if not already removed in Testo: migrate functional test (testcontainers) #37)
• Update README.md — replace PHPUnit references with Testo
• Verify no leftover PHPUnit imports or base classes remain in the codebase

Definition of Done

Testo is the only test framework. 20 tests green. No PHPUnit artifacts remain in the project (no config, no dependency, no references in CI/Makefile/README).

Blocked by: #37 · Part of #9

Branching Strategy

Create a branch from feature/testo and open a PR targeting feature/testo. Do not target main directly.