[ty] Preserve Self in `__new__` calls (#27003)
## Summary
Prior to this change, calling an inherited `__new__` implementation
could lose `Self` and return `Unknown`. This caused us to miss invalid
attribute access and broke chains of custom `__new__` implementations:
```py
from typing import Self
class Foo: ...
class Bar(Foo):
def __new__(cls) -> Self:
return Foo.__new__(cls)
class Baz(Bar):
def __new__(cls) -> Self:
result = Bar.__new__(cls) # Previously: Unknown
result.nonexistent() # Previously: no diagnostic
return result
```
This gives every unannotated `__new__` receiver an implicit
`type[Self]`, matching the constructor typing specification and
preserving `Self` through inherited calls.
It also preserves existing generic constructor calls where the synthetic
`cls` receiver would otherwise be specialized and checked twice. The
regression coverage includes overloaded callbacks, signature-preserving
and callable-protocol decorators, `Concatenate`, and aliased receiver
types; the intentionally odd nested and `Concatenate` inference results
remain documented as pre-existing limitations.
Closes https://github.com/astral-sh/ty/issues/3965.
---------
Co-authored-by: Carl Meyer <carl@astral.sh>
