Fix import ordering in service module

[brothercorvo]

Motivation

• Address lint warnings about module-level imports not being at the top of the file in reticulum_openapi/service.py.
• Ensure the side-effect-only import for multiprocessing compatibility is retained while keeping the import block contiguous.

Description

• Moved the from . import _multiprocessing_compat as _mp_compat import into the main import block so module-level imports are contiguous.
• Relocated the side-effect retention assignment _ = _mp_compat to the end of the import section to preserve the import’s side effects for linters.
• No functional logic was changed; only import ordering and the position of the side-effect assignment were updated in service.py.

Testing

• No automated tests were executed as part of this change.
• Static linting (flake8) was the motivating check for the edit, but it was not re-run here.
• Unit tests were not run.
• Integration tests were not run.

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Codex Task