test: improve temperature_utils.py coverage from 63% to 95%+

[Orinks]

Problem

temperature_utils.py currently has only 63% test coverage. The uncovered lines include:

• _normalize_dewpoint_unit() string normalization paths (line 72-79)
• calculate_dewpoint() edge cases: non-numeric inputs, zero humidity, Celsius mode (lines 108-110, 114, 132-133)
• format_temperature() with temperature_c only, smart precision logic, BOTH unit mode (lines 167, 171, 184-185, 190-193)
• get_temperature_values() various None combinations (lines 212-221)

Solution

Add comprehensive unit tests covering all branches and edge cases.

Acceptance Criteria

• Coverage of temperature_utils.py reaches 95%+
• All existing tests continue to pass

[Orinks]

Covered by PR #263