[codex] Add state estimation interface

[skyswordw]

Summary

• add PowerGridModelInterface.calculate_state_estimation() as a thin wrapper around PowerGridModel.calculate_state_estimation()
• store and return state-estimation output in output_data, matching the existing power-flow interface pattern
• add an integration test using a simple grid with voltage and power sensors

Closes #20

Validation

• uv run pytest tests/integration/loadflow/test_power_grid_model.py -q
• uv run ruff format --check src/power_grid_model_ds/_core/power_grid_model_interface.py tests/integration/loadflow/test_power_grid_model.py
• uv run ruff check src/power_grid_model_ds/_core/power_grid_model_interface.py tests/integration/loadflow/test_power_grid_model.py
• git diff --check
• uv run python -m compileall -q src/power_grid_model_ds tests/integration/loadflow/test_power_grid_model.py
• uv run pytest -q