[ty] The runtime object typing.Protocol is an instance of _ProtocolMeta (#20488)

## Summary

Fixes https://github.com/astral-sh/ty/issues/1218.

This bug doesn't currently cause us any real-world issues, because we
don't yet understand the signatures typeshed gives us for `isinstance()`
and `issubclass()` (typeshed's annotations there use PEP-613 type
aliases). #20107 demonstrates that this will start causing us issues as
soon as we add support for PEP-613 aliases, however, so it makes sense
to fix it now.

## Test Plan

Added mdtests

crates/ty_python_semantic/resources/mdtest/protocols.md

@@ -264,9 +264,21 @@ def f(
 # fmt: on
 ```
 
-Nonetheless, `Protocol` can still be used as the second argument to `issubclass()` at runtime:
+Nonetheless, `Protocol` is an instance of `type` at runtime, and therefore can still be used as the
+second argument to `issubclass()` at runtime:
 
 ```py
+import abc
+import typing
+from ty_extensions import TypeOf
+
+reveal_type(type(Protocol))  # revealed: <class '_ProtocolMeta'>
+# revealed: tuple[<class '_ProtocolMeta'>, <class 'ABCMeta'>, <class 'type'>, <class 'object'>]
+reveal_type(type(Protocol).__mro__)
+static_assert(is_subtype_of(TypeOf[Protocol], type))
+static_assert(is_subtype_of(TypeOf[Protocol], abc.ABCMeta))
+static_assert(is_subtype_of(TypeOf[Protocol], typing._ProtocolMeta))
+
 # Could also be `Literal[True]`, but `bool` is fine:
 reveal_type(issubclass(MyProtocol, Protocol))  # revealed: bool
 ```

crates/ty_python_semantic/src/types/class.rs

@@ -3694,6 +3694,7 @@ pub enum KnownClass {
     ParamSpec,
     ParamSpecArgs,
     ParamSpecKwargs,
+    ProtocolMeta,
     TypeVarTuple,
     TypeAliasType,
     NoDefaultType,
@@ -3821,6 +3822,7 @@ impl KnownClass {
             | Self::NamedTupleFallback
             | Self::NamedTupleLike
             | Self::ConstraintSet
+            | Self::ProtocolMeta
             | Self::TypedDictFallback => Some(Truthiness::Ambiguous),
 
             Self::Tuple => None,
@@ -3903,6 +3905,7 @@ impl KnownClass {
             | KnownClass::ConstraintSet
             | KnownClass::TypedDictFallback
             | KnownClass::BuiltinFunctionType
+            | KnownClass::ProtocolMeta
             | KnownClass::Template => false,
         }
     }
@@ -3982,6 +3985,7 @@ impl KnownClass {
             | KnownClass::ConstraintSet
             | KnownClass::TypedDictFallback
             | KnownClass::BuiltinFunctionType
+            | KnownClass::ProtocolMeta
             | KnownClass::Template => false,
         }
     }
@@ -4060,6 +4064,7 @@ impl KnownClass {
             | KnownClass::NamedTupleFallback
             | KnownClass::ConstraintSet
             | KnownClass::BuiltinFunctionType
+            | KnownClass::ProtocolMeta
             | KnownClass::Template => false,
         }
     }
@@ -4151,6 +4156,7 @@ impl KnownClass {
             | Self::ConstraintSet
             | Self::TypedDictFallback
             | Self::BuiltinFunctionType
+            | Self::ProtocolMeta
             | Self::Template => false,
         }
     }
@@ -4250,6 +4256,7 @@ impl KnownClass {
             Self::ConstraintSet => "ConstraintSet",
             Self::TypedDictFallback => "TypedDictFallback",
             Self::Template => "Template",
+            Self::ProtocolMeta => "_ProtocolMeta",
         }
     }
 
@@ -4477,6 +4484,7 @@ impl KnownClass {
             | Self::StdlibAlias
             | Self::Iterable
             | Self::Iterator
+            | Self::ProtocolMeta
             | Self::SupportsIndex => KnownModule::Typing,
             Self::TypeAliasType
             | Self::TypeVarTuple
@@ -4596,6 +4604,7 @@ impl KnownClass {
             | Self::ConstraintSet
             | Self::TypedDictFallback
             | Self::BuiltinFunctionType
+            | Self::ProtocolMeta
             | Self::Template => Some(false),
 
             Self::Tuple => None,
@@ -4680,6 +4689,7 @@ impl KnownClass {
             | Self::ConstraintSet
             | Self::TypedDictFallback
             | Self::BuiltinFunctionType
+            | Self::ProtocolMeta
             | Self::Template => false,
         }
     }
@@ -4773,6 +4783,7 @@ impl KnownClass {
             "ConstraintSet" => Self::ConstraintSet,
             "TypedDictFallback" => Self::TypedDictFallback,
             "Template" => Self::Template,
+            "_ProtocolMeta" => Self::ProtocolMeta,
             _ => return None,
         };
 
@@ -4855,9 +4866,9 @@ impl KnownClass {
             | Self::TypeVarTuple
             | Self::Iterable
             | Self::Iterator
+            | Self::ProtocolMeta
             | Self::NewType => matches!(module, KnownModule::Typing | KnownModule::TypingExtensions),
             Self::Deprecated => matches!(module, KnownModule::Warnings | KnownModule::TypingExtensions),
-
         }
     }
 

crates/ty_python_semantic/src/types/special_form.rs

@@ -160,9 +160,13 @@ impl SpecialFormType {
             | Self::Bottom
             | Self::Intersection
             | Self::CallableTypeOf
-            | Self::Protocol  // actually `_ProtocolMeta` at runtime but this is what typeshed says
             | Self::ReadOnly => KnownClass::SpecialForm,
 
+            // Typeshed says it's an instance of `_SpecialForm`,
+            // but then we wouldn't recognise things like `issubclass(`X, Protocol)`
+            // as being valid.
+            Self::Protocol => KnownClass::ProtocolMeta,
+
             Self::Generic | Self::Any => KnownClass::Type,
 
             Self::List