fix: annotate context managers with Generator for ty 0.0.85

[lesnik512]

Summary

Closes #159

ty 0.0.85 bundles a typeshed that deprecates the contextmanager / asynccontextmanager overloads for functions annotated -> Iterator[T] / -> AsyncIterator[T], so ty check in the scheduled run reported 9 warnings and failed. The context managers are now annotated -> Generator[T] / -> AsyncGenerator[T].

The types come from collections.abc, not typing: before 3.13, typing.Generator needs all three type arguments.

Changes

• src/httpware/_internal/exception_mapping.py: Generator / AsyncGenerator return types on the two exception mappers.
• src/httpware/client.py: same for _stream, _stream_capped, their async versions, and both stream() methods.
• tests/test_client_dispatch.py: same for _decode_spies.

Checklist

• just lint-ci passes with ty 0.0.85 (failed with 9 warnings before the change)
• just test passes (1007 tests)