[ty] Allow declared-only class-level attributes to be accessed on the class (#19071)

## Summary

Allow declared-only class-level attributes to be accessed on the class:
```py
class C:
    attr: int

C.attr  # this is now allowed
``` 

closes https://github.com/astral-sh/ty/issues/384
closes https://github.com/astral-sh/ty/issues/553

## Ecosystem analysis


* We see many removed `unresolved-attribute` false-positives for code
that makes use of sqlalchemy, as expected (see changes for `prefect`)
* We see many removed `call-non-callable` false-positives for uses of
`pytest.skip` and similar, as expected
* Most new diagnostics seem to be related to cases like the following,
where we previously inferred `int` for `Derived().x`, but now we infer
`int | None`. I think this should be a
conflicting-declarations/bad-override error anyway? The new behavior may
even be preferred here?
  ```py
  class Base:
      x: int | None
  
  
  class Derived(Base):
      def __init__(self):
          self.x: int = 1
  ```

crates/ty_python_semantic/resources/mdtest/attributes.md

@@ -87,13 +87,8 @@ c_instance = C()
 
 reveal_type(c_instance.declared_and_bound)  # revealed: str | None
 
-# Note that both mypy and pyright show no error in this case! So we may reconsider this in
-# the future, if it turns out to produce too many false positives. We currently emit:
-# error: [unresolved-attribute] "Attribute `declared_and_bound` can only be accessed on instances, not on the class object `<class 'C'>` itself."
-reveal_type(C.declared_and_bound)  # revealed: Unknown
+reveal_type(C.declared_and_bound)  # revealed: str | None
 
-# Same as above. Mypy and pyright do not show an error here.
-# error: [invalid-attribute-access] "Cannot assign to instance attribute `declared_and_bound` from the class object `<class 'C'>`"
 C.declared_and_bound = "overwritten on class"
 
 # error: [invalid-assignment] "Object of type `Literal[1]` is not assignable to attribute `declared_and_bound` of type `str | None`"
@@ -102,8 +97,11 @@ c_instance.declared_and_bound = 1
 
 #### Variable declared in class body and not bound anywhere
 
-If a variable is declared in the class body but not bound anywhere, we still consider it a pure
-instance variable and allow access to it via instances.
+If a variable is declared in the class body but not bound anywhere, we consider it to be accessible
+on instances and the class itself. It would be more consistent to treat this as a pure instance
+variable (and require the attribute to be annotated with `ClassVar` if it should be accessible on
+the class as well), but other type checkers allow this as well. This is also heavily relied on in
+the Python ecosystem:
 
 ```py
 class C:
@@ -113,11 +111,8 @@ c_instance = C()
 
 reveal_type(c_instance.only_declared)  # revealed: str
 
-# Mypy and pyright do not show an error here. We treat this as a pure instance variable.
-# error: [unresolved-attribute] "Attribute `only_declared` can only be accessed on instances, not on the class object `<class 'C'>` itself."
-reveal_type(C.only_declared)  # revealed: Unknown
+reveal_type(C.only_declared)  # revealed: str
 
-# error: [invalid-attribute-access] "Cannot assign to instance attribute `only_declared` from the class object `<class 'C'>`"
 C.only_declared = "overwritten on class"
 ```
 
@@ -1235,6 +1230,16 @@ def _(flag: bool):
     reveal_type(Derived().x)  # revealed: int | Any
 
     Derived().x = 1
+
+    # TODO
+    # The following assignment currently fails, because we first check if "a" is assignable to the
+    # attribute on the meta-type of `Derived`, i.e. `<class 'Derived'>`. When accessing the class
+    # member `x` on `Derived`, we only see the `x: int` declaration and do not union it with the
+    # type of the base class attribute `x: Any`. This could potentially be improved. Note that we
+    # see a type of `int | Any` above because we have the full union handling of possibly-unbound
+    # *instance* attributes.
+
+    # error: [invalid-assignment] "Object of type `Literal["a"]` is not assignable to attribute `x` of type `int`"
     Derived().x = "a"
 ```
 
@@ -1299,10 +1304,8 @@ def _(flag: bool):
             if flag:
                 self.x = 1
 
-    # error: [possibly-unbound-attribute]
     reveal_type(Foo().x)  # revealed: int | Unknown
 
-    # error: [possibly-unbound-attribute]
     Foo().x = 1
 ```
 

crates/ty_python_semantic/resources/mdtest/call/dunder.md

@@ -120,8 +120,7 @@ def _(flag: bool):
 
 ### Dunder methods as class-level annotations with no value
 
-Class-level annotations with no value assigned are considered instance-only, and aren't available as
-dunder methods:
+Class-level annotations with no value assigned are considered to be accessible on the class:
 
 ```py
 from typing import Callable
@@ -129,10 +128,8 @@ from typing import Callable
 class C:
     __call__: Callable[..., None]
 
-# error: [call-non-callable]
 C()()
 
-# error: [invalid-assignment]
 _: Callable[..., None] = C()
 ```
 

crates/ty_python_semantic/resources/mdtest/dataclasses.md

@@ -810,21 +810,6 @@ D(1)  # OK
 D()  # error: [missing-argument]
 ```
 
-### Accessing instance attributes on the class itself
-
-Just like for normal classes, accessing instance attributes on the class itself is not allowed:
-
-```py
-from dataclasses import dataclass
-
-@dataclass
-class C:
-    x: int
-
-# error: [unresolved-attribute] "Attribute `x` can only be accessed on instances, not on the class object `<class 'C'>` itself."
-C.x
-```
-
 ### Return type of `dataclass(...)`
 
 A call like `dataclass(order=True)` returns a callable itself, which is then used as the decorator.

crates/ty_python_semantic/resources/mdtest/protocols.md

@@ -533,7 +533,13 @@ class FooSubclassOfAny:
     x: SubclassOfAny
 
 static_assert(not is_subtype_of(FooSubclassOfAny, HasX))
-static_assert(not is_assignable_to(FooSubclassOfAny, HasX))
+
+# `FooSubclassOfAny` is assignable to `HasX` for the following reason. The `x` attribute on `FooSubclassOfAny`
+# is accessible on the class itself. When accessing `x` on an instance, the descriptor protocol is invoked, and
+# `__get__` is looked up on `SubclassOfAny`. Every member access on `SubclassOfAny` yields `Any`, so `__get__` is
+# also available, and calling `Any` also yields `Any`. Thus, accessing `x` on an instance of `FooSubclassOfAny`
+# yields `Any`, which is assignable to `int` and vice versa.
+static_assert(is_assignable_to(FooSubclassOfAny, HasX))
 
 class FooWithY(Foo):
     y: int
@@ -1586,11 +1592,7 @@ def g(a: Truthy, b: FalsyFoo, c: FalsyFooSubclass):
     reveal_type(bool(c))  # revealed: Literal[False]
 ```
 
-It is not sufficient for a protocol to have a callable `__bool__` instance member that returns
-`Literal[True]` for it to be considered always truthy. Dunder methods are looked up on the class
-rather than the instance. If a protocol `X` has an instance-attribute `__bool__` member, it is
-unknowable whether that attribute can be accessed on the type of an object that satisfies `X`'s
-interface:
+The same works with a class-level declaration of `__bool__`:
 
 ```py
 from typing import Callable
@@ -1599,7 +1601,7 @@ class InstanceAttrBool(Protocol):
     __bool__: Callable[[], Literal[True]]
 
 def h(obj: InstanceAttrBool):
-    reveal_type(bool(obj))  # revealed: bool
+    reveal_type(bool(obj))  # revealed: Literal[True]
 ```
 
 ## Callable protocols
@@ -1832,7 +1834,8 @@ def _(r: Recursive):
     reveal_type(r.direct)  # revealed: Recursive
     reveal_type(r.union)  # revealed: None | Recursive
     reveal_type(r.intersection1)  # revealed: C & Recursive
-    reveal_type(r.intersection2)  # revealed: C & ~Recursive
+    # revealed: @Todo(map_with_boundness: intersections with negative contributions) | (C & ~Recursive)
+    reveal_type(r.intersection2)
     reveal_type(r.t)  # revealed: tuple[int, tuple[str, Recursive]]
     reveal_type(r.callable1)  # revealed: (int, /) -> Recursive
     reveal_type(r.callable2)  # revealed: (Recursive, /) -> int

crates/ty_python_semantic/resources/mdtest/type_qualifiers/classvar.md

@@ -64,24 +64,6 @@ c = C()
 c.a = 2
 ```
 
-and similarly here:
-
-```py
-class Base:
-    a: ClassVar[int] = 1
-
-class Derived(Base):
-    if flag():
-        a: int
-
-reveal_type(Derived.a)  # revealed: int
-
-d = Derived()
-
-# error: [invalid-attribute-access]
-d.a = 2
-```
-
 ## Too many arguments
 
 ```py