test: kill convert_temp rounding precision mutant

[Copilot]

Mutation testing identified a surviving mutant in convert_temp: changing round(..., 1) to round(..., 2) was undetected because all existing Fahrenheit test inputs produced results with ≤1 significant decimal place, making both rounding depths equivalent.

Change

• tests/test_models.py: Add test_fahrenheit_rounding_precision to TestConvertTemp using 22.3°C as input

def test_fahrenheit_rounding_precision(self):
    """22.3°C -> 72.14°F: round to 1 decimal gives 72.1, not 72.14."""
    assert convert_temp(22.3, TempUnit.FAHRENHEIT) == 72.1

22.3°C → 72.14°F: round(72.14, 1) = 72.1 vs round(72.14, 2) = 72.14 — the two rounding depths now produce distinguishable results.

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