feat: added a fallback to class docstring

[jny0444]

Hovering a constructor call now shows the class docstring when __init__ or __new__ has none.

Fixes #5254

Calling a class, as in foo = MyClass(bar), resolves hover to __init__ or __new__. Those methods often have no docstring, so the tooltip was empty even when the class documented itself. Hover still prefers a nonempty method docstring. When that docstring is missing or blank, it uses the class docstring instead, preferring a stub over the implementation. Go-to-definition still lands on the method.

Test plan

• cargo test -p pyrefly --lib test::lsp::hover::
• Covered __init__ with no docstring, __new__ with no docstring, and the case where neither the method nor the class has a docstring.

[meta-codesync]

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