refactor: _safe_fetch helper + price-sensor MRO cleanup

[mrebbert]

Summary

Two-candidate refactor sweep from the post-0.1.53 audit — both preserve behaviour, both fully covered by the existing tests.

• _safe_fetch(label, fn) in `init.py`: folded the three copies of the setup-time `try SDK call; except → warn + return None` shape into one helper. Applied to `get_details`, `get_subscriptions`, `get_impact_overview`. Net −9 lines; the next setup-time fetch is one line, not five.
• OneKomma5PriceSensor MRO: added `_DescriptionValueSensor` to the MRO and dropped the byte-equivalent `native_value` override. The custom `extra_state_attributes` (only fires for `current_electricity_price`) stays because the mixin's default returns None. Net −6 lines, and no more "why does this class copy the mixin?" reader stumble.

Test plan

• Full suite green (`.venv/bin/pytest -q` → 226 passed).
• mypy + ruff clean.
• Live-deployed on homie; smoke-tested three sensors that exercise both changes: `sensor.heizungskeller_1k5_tagesersparnis` still carries `co2_saved_kg`, `sensor.1k5_aktueller_strompreis` still has state + full forecast/grid-cost attributes, `sensor.markus_rebbert_dynamic_pulse_price_guarantee` still resolves.

Rejected from the same audit (memory in `~/.claude/projects/-Users-markus-Sources-1komma5-ha/`): base-coordinator `_on_data` hook (bigger blast radius, better as its own PR); EV entity ctor consolidation (blast across 5 modules, defer).