[ty] Exhaustiveness checking for generic classes (#21726)

## Summary

We had tests for this already, but they used generic classes that were
bivariant in their type parameter, and so this case wasn't captured.

closes https://github.com/astral-sh/ty/issues/1702

## Test Plan

Updated Markdown tests

crates/ty_python_semantic/resources/mdtest/exhaustiveness_checking.md

@@ -284,10 +284,17 @@ python-version = "3.12"
 ```py
 from typing import assert_never
 
-class A[T]: ...
+class A[T]:
+    value: T
+
 class ASub[T](A[T]): ...
-class B[T]: ...
-class C[T]: ...
+
+class B[T]:
+    value: T
+
+class C[T]:
+    value: T
+
 class D: ...
 class E: ...
 class F: ...

crates/ty_python_semantic/src/semantic_index/reachability_constraints.rs

@@ -336,6 +336,7 @@ fn pattern_kind_to_type<'db>(db: &'db dyn Db, kind: &PatternPredicateKind<'db>)
                 infer_expression_type(db, *class_expr, TypeContext::default())
                     .to_instance(db)
                     .unwrap_or(Type::Never)
+                    .top_materialization(db)
             } else {
                 Type::Never
             }