[ty] Add generic inference for dataclasses (#18443)

## Summary

An issue seen here https://github.com/astral-sh/ty/issues/500

The `__init__` method of dataclasses had no inherited generic context,
so we could not infer the type of an instance from a constructor call
with generics

## Test Plan

Add tests to classes.md` in generics folder

crates/ty_python_semantic/src/types/class.rs

@@ -1372,7 +1372,8 @@ impl<'db> ClassLiteral<'db> {
                 parameters.push(parameter);
             }
 
-            let signature = Signature::new(Parameters::new(parameters), Some(Type::none(db)));
+            let mut signature = Signature::new(Parameters::new(parameters), Some(Type::none(db)));
+            signature.inherited_generic_context = self.generic_context(db);
             Some(CallableType::function_like(db, signature))
         };