fix(river2): support River 2 family telemetry and offline states

[Komzpa]

Summary

This PR narrows the remaining River 2 work to device-family telemetry and
offline-state handling.

It is now rebased on current main, after the MQTT reconnect/status-id fixes
from #787 and #789. Those fixes are intentionally not duplicated here. This PR
does not replace the MQTT client; it keeps the upstream MQTT path from main.

What changes

• Read River 2 and River 2 Max solar current/voltage from live mppt.*
  telemetry while preserving the existing inv.dcIn* entity IDs.
• Add matching River 2 Pro solar current/voltage support.
• Expose River 2 family DC mode and MPPT fault diagnostics.
• Remove the generic per-device Reconnect button while keeping
  device-specific buttons.
• Mark integration-derived energy sensors as total for Home Assistant Energy
  compatibility.
• Make data entities unavailable on explicit offline status instead of
  reporting stale/default 0 values.
• Propagate /device/list status-poll results into per-device coordinators so
  explicit offline status updates HA availability.

Notes for review

• MQTT reconnect handling is already in fix(mqtt): reconnect after dropped broker connection #787.
• Scheduled status entity ID stability is already in fix(status): keep scheduled status entity id stable #789.
• ASSUME_OFFLINE remains a soft "silent for now" state; only explicit
  OFFLINE hides telemetry.
• River 2 solar entity IDs stay stable: entity_key preserves the old
  inv.dcIn* unique IDs while values are sourced from mppt.*.

Validation

• python3 -m ruff check custom_components/ecoflow_cloud/api/ecoflow_mqtt.py custom_components/ecoflow_cloud/api/__init__.py custom_components/ecoflow_cloud/devices/data_holder.py custom_components/ecoflow_cloud/devices/status_coordinator.py custom_components/ecoflow_cloud/entities/__init__.py custom_components/ecoflow_cloud/sensor.py custom_components/ecoflow_cloud/devices/internal/river2.py custom_components/ecoflow_cloud/devices/internal/river2_max.py custom_components/ecoflow_cloud/devices/internal/river2_pro.py
• python3 -m py_compile custom_components/ecoflow_cloud/api/ecoflow_mqtt.py custom_components/ecoflow_cloud/api/__init__.py custom_components/ecoflow_cloud/devices/data_holder.py custom_components/ecoflow_cloud/devices/status_coordinator.py custom_components/ecoflow_cloud/entities/__init__.py custom_components/ecoflow_cloud/sensor.py custom_components/ecoflow_cloud/devices/internal/river2.py custom_components/ecoflow_cloud/devices/internal/river2_max.py custom_components/ecoflow_cloud/devices/internal/river2_pro.py
• git diff --check
• GitHub checks are passing on the current head: HACS validation, lint, and
  hassfest validation.

[Komzpa]

@tolwi running this one for a while already, solar voltage/current fix from here is really handy - anything else I need to fix here before merging?