Support validation rule autocompletion in form requests

[ace-of-aces]

Closes #291, #474

This adds support for autocompletion inside the rules method of form requests.

Specifically, this implements the necessary logic to the completion registry for the classExtends and methodDefinition fields, which are already defined (but unused) in the tags() function of the validation completion provider.

Note

I implemented this with the help of an AI agent (Claude Opus 4.5). I tested this and reviewed the code, which I deemed to be consistent with other parts in the completion provider.

Also, I corrected the completion template for the after rule, which was missing the template braces around the placeholder.

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Screenshots

Before:

After:

[N1ebieski]

If accepted, this can be closed: #336

[ace-of-aces]

Hey @N1ebieski, sorry that I just didn't find your open PR when doing a quick search😅

Happy with whatever PR will end up getting merged as they're basically doing the same with some minor differences :)