[ty] Fix __setattr__ call check precedence during attribute assignment (#18347)

## Summary

Related:

- astral-sh/ty#111
- #17974 (comment)

Previously, when validating an attribute assignment, a `__setattr__`
call check was only done if the attribute wasn't found as either a class
member or instance member

This PR changes the `__setattr__` call check to be attempted first,
prior to the "[normal
mechanism](https://docs.python.org/3/reference/datamodel.html#object.__setattr__)",
as a defined `__setattr__` should take precedence over setting an
attribute on the instance dictionary directly.

if the return type of `__setattr__` is `Never`, an `invalid-assignment`
diagnostic is emitted

Once this is merged, a subsequent PR will synthesize a `__setattr__`
method with a `Never` return type for frozen dataclasses.

## Test Plan

Existing tests + mypy_primer

---------

Co-authored-by: David Peter <mail@david-peter.de>

Author: thejchap

crates/ty_python_semantic/resources/mdtest/attributes.md

@@ -1791,6 +1791,80 @@ date.year = 2025
 date.tz = "UTC"
 ```
 
+### Return type of `__setattr__`
+
+If the return type of the `__setattr__` method is `Never`, we do not allow any attribute assignments
+on instances of that class:
+
+```py
+from typing_extensions import Never
+
+class Frozen:
+    existing: int = 1
+
+    def __setattr__(self, name, value) -> Never:
+        raise AttributeError("Attributes can not be modified")
+
+instance = Frozen()
+instance.non_existing = 2  # error: [invalid-assignment] "Cannot assign to attribute `non_existing` on type `Frozen` whose `__setattr__` method returns `Never`/`NoReturn`"
+instance.existing = 2  # error: [invalid-assignment] "Cannot assign to attribute `existing` on type `Frozen` whose `__setattr__` method returns `Never`/`NoReturn`"
+```
+
+### `__setattr__` on `object`
+
+`object` has a custom `__setattr__` implementation, but we still emit an error if a non-existing
+attribute is assigned on an `object` instance.
+
+```py
+obj = object()
+obj.non_existing = 1  # error: [unresolved-attribute]
+```
+
+### Setting attributes on `Never` / `Any`
+
+Setting attributes on `Never` itself should be allowed (even though it has a `__setattr__` attribute
+of type `Never`):
+
+```py
+from typing_extensions import Never, Any
+
+def _(n: Never):
+    reveal_type(n.__setattr__)  # revealed: Never
+
+    # No error:
+    n.non_existing = 1
+```
+
+And similarly for `Any`:
+
+```py
+def _(a: Any):
+    reveal_type(a.__setattr__)  # revealed: Any
+
+    # No error:
+    a.non_existing = 1
+```
+
+### Possibly unbound `__setattr__` method
+
+If a `__setattr__` method is only partially bound, the behavior is still the same:
+
+```py
+from typing_extensions import Never
+
+def flag() -> bool:
+    return True
+
+class Frozen:
+    if flag():
+        def __setattr__(self, name, value) -> Never:
+            raise AttributeError("Attributes can not be modified")
+
+instance = Frozen()
+instance.non_existing = 2  # error: [invalid-assignment]
+instance.existing = 2  # error: [invalid-assignment]
+```
+
 ### `argparse.Namespace`
 
 A standard library example of a class with a custom `__setattr__` method is `argparse.Namespace`: