Add unit tests for System\Telegram support utilities

[ismailian]

Motivation

• Provide unit coverage for Telegram support utilities to catch regressions in entity building and hydration logic.
• Verify EntityBuilder behavior for chained entity construction, optional fields, and textMention serialization.
• Ensure Hydrator correctly maps attribute-driven properties including nested objects, arrays, enums, DateTime conversion, and self-payload mapping.

Description

• Add tests/Unit/System/Telegram/Support/EntityBuilderTest.php covering insertion order, optional field omission, and textMention user serialization.
• Add tests/Unit/System/Telegram/Support/HydratorTest.php covering scalar mapping, object mapping, array-of-objects mapping, enum mapping, DateTime conversion, and MapProp(':', ...) self payload behavior.
• Add phpunit.xml test configuration and update composer.json with autoload-dev for Tests\, a test script (phpunit), and a require-dev entry for phpunit/phpunit.

Testing

• php -l tests/Unit/System/Telegram/Support/EntityBuilderTest.php and php -l tests/Unit/System/Telegram/Support/HydratorTest.php both reported no syntax errors.
• Attempt to install PHPUnit via composer update phpunit/phpunit --with-all-dependencies failed due to a network error (Packagist tunnel returned 403) in this environment.
• Running composer test could not execute because the phpunit binary was not available due to the failed dependency installation.

---

Codex Task