Fix heat-exchanger pre-commit failures

[Copilot]

The pre-commit job failed because isort and Ruff reformatted the heat-exchanger files, and Ruff flagged an unused unpacked variable.

• Hook compliance: Apply the reported formatting changes and discard the unused return value:

  _, Tc_, *_ = self.march_segments(...)

• Stream definitions: Add process- and working-fluid metadata required by the model.