fix(python-sdk): route order-book unwatch via HTTP

[realfishsam]

Summary

• Routes Python unwatch_order_book through the sidecar HTTP unwatchOrderBook endpoint, matching TypeScript and unwatch_address behavior.
• Updates the streaming transport regression test so order-book unwatch no longer requires an active WebSocket client.

Fixes #1315

Test Plan

• PYTHONPATH=/tmp/pmxt_test_stubs:sdks/python uv run --no-project --with pytest --with pydantic --with python-dateutil --with 'urllib3>=2.7.0' --with typing-extensions python -m pytest sdks/python/tests/test_streaming_ws_transport.py::test_unwatch_order_book_uses_http_sidecar_transport -q
• PYTHONPATH=/tmp/pmxt_test_stubs:sdks/python uv run --no-project --with pytest --with pydantic --with python-dateutil --with 'urllib3>=2.7.0' --with typing-extensions python -m pytest sdks/python/tests/test_streaming_ws_transport.py -q
• python3 -m py_compile sdks/python/pmxt/client.py sdks/python/tests/test_streaming_ws_transport.py

Note: the committed checkout does not include generated pmxt_internal artifacts, so local pytest used a temporary untracked minimal /tmp/pmxt_test_stubs/pmxt_internal stub for imports only; the exercised code path is the hand-maintained SDK wrapper.