fix: annotate context managers with Generator for ty 0.0.85

[lesnik512]

Summary

Fixes the scheduled CI failure. Closes #205

ty 0.0.85 ships a typeshed that deprecates the contextmanager and asynccontextmanager overloads for functions annotated -> Iterator[T] or -> AsyncIterator[T]. ty check reported 12 such warnings, which fail lint-ci.

Changes

• Annotate the 12 decorated functions in faststream_outbox/, tests/ and benchmarks/ as Generator[T] / AsyncGenerator[T].
• Import them from collections.abc. typing.Generator needs all three type arguments before Python 3.13.
• Keep Iterator / AsyncIterator imports only where other code still uses them.

Checklist

• Lint and format pass (ruff)
• Type check passes (ty)
• Tests pass and new behavior is covered
• Build succeeds (uv build) if packaging or build config changed
• Repo metadata stays consistent across the three surfaces (GitHub description, pyproject description, profile blurb) if this touches packaging

[github-actions]

Benchmark gate

✅ gate passed

scenario	msg/s	delete/msg	WALrec/msg	WALB/msg	fpi	upd	del	dead_tup
consumer/w1/b10	1002	1.000	6.79	920	242	5000	5000	10000
consumer/w1/b100	994	1.000	6.84	918	243	5000	5000	10000
consumer/w2/b10	1175	1.000	6.80	959	242	5000	5000	10000
consumer/w2/b100	1390	1.000	6.72	954	243	5000	5000	10000
consumer/w4/b10	1066	1.000	6.78	961	244	5000	5000	10000
consumer/w4/b100	1608	1.000	6.87	1034	243	5000	5000	10000
consumer/w1/b100/tfbs100	5605	0.010	6.07	1116	243	5000	5000	10000
producer/w1/b100	2233	0.000	3.04	584	0	0	0	0

Gated (fails the build): delete_calls + tuple counters (upd/del/ins) + the producer's insert_calls, exact; select_calls within +2; wal_records within a 10% band. msg/s, WAL bytes and total calls are informational (timing/FPI noise).