fix: skip blank stream response lines

[lin-hongkuan]

Summary

• Ignore blank lines while iterating sync and async streaming responses.
• Keep the existing JSON parsing behavior for non-empty stream payloads.

Why

Some HTTP streaming transports can yield heartbeat/empty lines. Those should not cause json.loads() to fail before the next valid response object arrives.

Testing

• python -m pytest tests/test_client.py::test_client_chat_stream_skips_blank_lines tests/test_client.py::test_async_client_chat_stream_skips_blank_lines tests/test_client.py::test_client_chat_stream tests/test_client.py::test_async_client_chat_stream -q
• git diff --check