[ty] Make dataclass instances adhere to DataclassInstance (#18115)

## Summary

Make dataclass instances adhere to the `DataclassInstance` protocol.

fixes astral-sh/ty#400

## Test Plan

New Markdown tests

crates/ty_python_semantic/resources/mdtest/dataclasses.md

@@ -618,21 +618,43 @@ To do
 
 ## `dataclass.fields`
 
-Dataclasses have `__dataclass_fields__` in them, which makes them a subtype of the
-`DataclassInstance` protocol.
-
-Here, we verify that dataclasses can be passed to `dataclasses.fields` without any errors, and that
-the return type of `dataclasses.fields` is correct.
+Dataclasses have a special `__dataclass_fields__` class variable member. The `DataclassInstance`
+protocol checks for the presence of this attribute. It is used in the `dataclasses.fields` and
+`dataclasses.asdict` functions, for example:
 
 ```py
-from dataclasses import dataclass, fields
+from dataclasses import dataclass, fields, asdict
 
 @dataclass
 class Foo:
     x: int
 
-reveal_type(Foo.__dataclass_fields__)  # revealed: dict[str, Field[Any]]
+foo = Foo(1)
+
+reveal_type(foo.__dataclass_fields__)  # revealed: dict[str, Field[Any]]
 reveal_type(fields(Foo))  # revealed: tuple[Field[Any], ...]
+reveal_type(asdict(foo))  # revealed: dict[str, Any]
+```
+
+The class objects themselves also have a `__dataclass_fields__` attribute:
+
+```py
+reveal_type(Foo.__dataclass_fields__)  # revealed: dict[str, Field[Any]]
+```
+
+They can be passed into `fields` as well, because it also accepts `type[DataclassInstance]`
+arguments:
+
+```py
+reveal_type(fields(Foo))  # revealed: tuple[Field[Any], ...]
+```
+
+But calling `asdict` on the class object is not allowed:
+
+```py
+# TODO: this should be a invalid-argument-type error, but we don't properly check the
+# types (and more importantly, the `ClassVar` type qualifier) of protocol members yet.
+asdict(Foo)
 ```
 
 ## Other special cases

crates/ty_python_semantic/src/types.rs

@@ -2957,19 +2957,6 @@ impl<'db> Type<'db> {
                 ))
                 .into()
             }
-            Type::ClassLiteral(class)
-                if name == "__dataclass_fields__" && class.dataclass_params(db).is_some() =>
-            {
-                // Make this class look like a subclass of the `DataClassInstance` protocol
-                Symbol::bound(KnownClass::Dict.to_specialized_instance(
-                    db,
-                    [
-                        KnownClass::Str.to_instance(db),
-                        KnownClass::Field.to_specialized_instance(db, [Type::any()]),
-                    ],
-                ))
-                .with_qualifiers(TypeQualifiers::CLASS_VAR)
-            }
             Type::BoundMethod(bound_method) => match name_str {
                 "__self__" => Symbol::bound(bound_method.self_instance(db)).into(),
                 "__func__" => {

crates/ty_python_semantic/src/types/class.rs

@@ -1132,6 +1132,18 @@ impl<'db> ClassLiteral<'db> {
         specialization: Option<Specialization<'db>>,
         name: &str,
     ) -> SymbolAndQualifiers<'db> {
+        if name == "__dataclass_fields__" && self.dataclass_params(db).is_some() {
+            // Make this class look like a subclass of the `DataClassInstance` protocol
+            return Symbol::bound(KnownClass::Dict.to_specialized_instance(
+                db,
+                [
+                    KnownClass::Str.to_instance(db),
+                    KnownClass::Field.to_specialized_instance(db, [Type::any()]),
+                ],
+            ))
+            .with_qualifiers(TypeQualifiers::CLASS_VAR);
+        }
+
         let body_scope = self.body_scope(db);
         let symbol = class_symbol(db, body_scope, name).map_type(|ty| {
             // The `__new__` and `__init__` members of a non-specialized generic class are handled