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Require the target in the proxies' __new__() (#1782)

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## What do these changes do?

`MultiDictProxy.__new__(MultiDictProxy)` (and the `CIMultiDictProxy`
equivalent) used `PyType_GenericNew`, so a proxy built without
`__init__()` kept `md == NULL`, and `p.get("k")`, `p["k"]`, `len(p)` or
the C API getters then crashed the interpreter. The proxies got their
own `tp_new`, which allocates and then runs the class's `tp_init`, so
`__new__()` takes the same single argument as the constructor and
rejects a missing or wrong-typed target. No proxy can exist without a
multidict behind it, so the forwarders and the C API need no NULL
checks.

The pure-Python backend got a matching `__new__()`; the target
validation moved into a `_set_target()` helper that `__new__()` and
`__init__()` share, with `CIMultiDictProxy` overriding it.

`tp_new` is `COLD` and calls `tp_init` through a pointer: exact-type
construction always goes through `tp_vectorcall`, so only subclasses and
explicit `__new__()` calls reach it. Without that, inlining
`_multidict_proxy_init()` into the two new functions pushed
`md_get_one()` out of `multidict_mp_subscript()` on 3.11 and out of
`multidict_proxy_mp_subscript()` on 3.14 (`getitem_hit` +7..9% on 3.11).

## Are there changes in behavior for the user?

Yes. `MultiDictProxy.__new__()` and `CIMultiDictProxy.__new__()` require
the proxied multidict instead of returning an object that crashes (C) or
raises `AttributeError` (pure Python) on use. A proxy subclass whose
`__init__()` takes different arguments than the base class now also
needs to override `__new__()`.

## Is it a substantial burden for the maintainers to support this?

No; three small functions in the C extension and one helper in the
pure-Python backend.

## Related issue number

N/A, no issue filed.

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
- [x] Documentation reflects the changes (N/A, no... (continued)

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