refactor: add assert on LiveModelTestCase::createModel()

[paulbalandan]

Description
Adds a @phpstan-assert to let phpstan know the type of $this->model after calling $this->createModel().

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide