test: raise gemini API client coverage to 90%

[Patrik652]

Summary

• expand tests/unit/test_gemini_api_client.py to cover config-key loading, validation/refresh behavior, model init path, and runtime error mapping
• harden Gemini error mapping with case-insensitive invalid-key detection
• keep existing API behavior while improving typed exception mapping coverage

Test Plan

• PYTHONPATH=. .venv/bin/ruff check src/services src/cli tests/unit/test_gemini_api_client.py
• PYTHONPATH=. .venv/bin/mypy src/services src/cli
• PYTHONPATH=. .venv/bin/pytest -q

Result

• 349 passed, 0 skipped, 2 warnings
• total coverage: 83%
• src/services/gemini_api_client.py: 90%