Fix getProviderClassName() return type

[felixarntz]

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[felixarntz]

@JasonTheAdams There's some weird PHPStan issue when I change it, see https://github.com/WordPress/php-ai-client/actions/runs/20122278394/job/57744921021?pr=147: I don't see why it complains about the value being a string here. As far as I can tell, the type annotations for the underlying class properties correctly use class-string<ProviderInterface>. Any idea what's going on? 🤔

[JasonTheAdams]

@felixarntz check out 30b3ffd

So the issue is that we're have a union string|class-string type being returned, and PHPStan isn't particularly good at following type narrowing by tracking the isset() logic the way we were going about it. So PHPStan was never confident when it logically should have been a class-string.

Fortunately, PHPStan has @phpstan-assert-if-true tags that we can use here which helps guide the static analysis system. Happily, it also makes our checks a bit more declarative. This now works! 😄

[JasonTheAdams]

Approving this, but you should as well, @felixarntz, since I ended up changing more code than you. Hahah!

[felixarntz]

@JasonTheAdams Awesome! Really appreciate learning more gotchas about PHPStan from you :)