Add a native OutOfOrderTableProxy.__contains__

[tfoutrein]

What

OutOfOrderTableProxy was the last mapping type still inheriting the MutableMapping.__contains__ mixin. That mixin implements key in proxy as try: proxy[key] — routing through __getitem__, which resolves the value (and builds a NonExistentKey on every absent key) only to throw it away.

Add a native __contains__:

def __contains__(self, key: object) -> bool:
    return key in self._internal_container

This is exactly the predicate __getitem__ already uses as its guard (if key not in self._internal_container: raise NonExistentKey), and it is itself a native Container.__contains__ — which still rebuilds the proxy for an out-of-order key so its validation runs exactly as before.

This completes #483, which added native __contains__ to Container, Table and InlineTable.

Iso-functional

Both the old (mixin) and new path bottom out on the same Container.__contains__, so the membership result, the raised exception type and document (non-)mutation are identical. Verified by:

• the full test suite incl. the toml-test conformance submodule;
• a new focused test exercising a real OutOfOrderTableProxy (present / absent / Key-object / non-str→TypeError / no document mutation);
• a differential over many out-of-order / nested-proxy / dotted-key / AoT documents and post-mutation states — membership result, exception type and as_string() all byte-identical to master.

Benchmark

Membership on an out-of-order proxy (median, interleaved A/B vs master):

	speedup
present key	~2.1×
absent key	~1.3×

The parse path is untouched (no measurable change there).