Fix some phpstan warnings

[norkunas]

Summary by CodeRabbit

• Bug Fixes

  • Fixed incorrect HTTP response header content-type format in test fixtures.

• Chores

  • Updated development dependencies to newer patch releases.
  • Refined CI configuration for code style checks.
  • Reworked tests and test fixtures for stronger typing, nullable/readonly properties, and improved assertions to enhance reliability.

_{✏️ Tip: You can customize this high-level summary in your review settings.}