[ty] route constructor calls through bindings (#22124)
## Summary
fixes https://github.com/astral-sh/ty/issues/2124
* route constructor calls through the existing `Type::bindings`
machinery via `constructor_bindings()`, ensuring constructor return
types are instance-typed (not `__init__`'s `None`)
* delete the legacy constructor call path
* constructor overload resolution is deterministic (first matching
overload), and implicit `__new__`/`__init__` lints remain
method-specific and fire once per call site (including the
missing-`__init__` fallback)
## Test Plan
* mdtest (constructor): added coverage for `@staticmethod __new__`,
conflicting `__new__`/`__init__` parameter types, `__new__`-only
fallback (object `__init__` accepts args), and generic constructor
inference.
* mdtest (decorators): `functools.cached_property` specifically added to
address https://github.com/astral-sh/ty/issues/1446
* mdtest (classes): fixed the TODOs for generic class params
`C[Unknown]` are now correctly revealed as `C[int]` (and constructor
specialization uses first matching overload)
* mdtest (functions): improved constructor path now correctly surfaces
both the ill-typed generic call and the resulting invalid assignment
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Co-authored-by: Carl Meyer <carl@astral.sh>
