Improve PHP 8.5+ support and update test environment

[clue]

Builds on top of #47, #49, #26, reactphp/promise#264, reactphp/event-loop#282, and clue/framework-x#297

[Copilot]

Pull request overview

Updates dependencies and CI to better accommodate newer PHP versions (PHP 8.5+) while tightening SSE retry parsing behavior and adjusting tests for newer PHP reflection behavior.

Changes:

• Extend CI matrix to include PHP 8.5 and update the checkout action version.
• Tighten retry: field parsing rules in MessageEvent::parse() and expand coverage for edge cases.
• Adjust reflection usage in tests to avoid PHP 8.1+ setAccessible() behavior changes.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/MessageEvent.php	Updates validation logic for parsing retry: values.
tests/MessageEventTest.php	Adds more invalid retry: cases to the data provider.
tests/EventSourceTest.php	Conditionally avoids ReflectionProperty::setAccessible() for PHP 8.1+.
composer.json	Updates ReactPHP dependency constraints.
.github/workflows/ci.yml	Adds PHP 8.5 to CI matrix and updates actions/checkout major version.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.