[red-knot] Understand type[Unknown] (#15409)

## Summary

Follow-up to #15194.

## Test Plan

Markdown tests.

crates/red_knot_python_semantic/resources/mdtest/directives/assert_type.md

@@ -66,14 +66,11 @@ def _(a: Unknown, b: Any):
     assert_type(b, Unknown)  # fine
 
 def _(a: type[Unknown], b: type[Any]):
-    # TODO: Should be `type[Unknown]`
-    reveal_type(a)  # revealed: @Todo(unsupported type[X] special form)
-    # TODO: Should be fine
-    assert_type(a, type[Any])  # error: [type-assertion-failure]
+    reveal_type(a)  # revealed: type[Unknown]
+    assert_type(a, type[Any])  # fine
 
     reveal_type(b)  # revealed: type[Any]
-    # TODO: Should be fine
-    assert_type(b, type[Unknown])  # error: [type-assertion-failure]
+    assert_type(b, type[Unknown])  # fine
 ```
 
 ## Tuples

crates/red_knot_python_semantic/src/types/infer.rs

@@ -5066,6 +5066,9 @@ impl<'db> TypeInferenceBuilder<'db> {
                     Type::KnownInstance(KnownInstanceType::Any) => {
                         SubclassOfType::subclass_of_any()
                     }
+                    Type::KnownInstance(KnownInstanceType::Unknown) => {
+                        SubclassOfType::subclass_of_unknown()
+                    }
                     _ => todo_type!("unsupported type[X] special form"),
                 }
             }