Handle unknown OffReason and AlarmReason values gracefully

[rajlaud]

Problem

When a Victron device sends an enum value that isn't defined in the library (e.g., a new OffReason or AlarmReason value from a firmware update), the library raises a ValueError. This causes the entire advertisement to be dropped, losing all sensor data from that reading.

This became more likely after #74 changed OffReason and AlarmReason from Flag to Enum, since previously unknown combinations could still be represented as flag combinations.

Solution

Added a parse_enum() helper function and UNKNOWN members to OffReason and AlarmReason. When an unrecognized value is encountered, the enum now returns UNKNOWN instead of raising an exception.

Changes

• Added parse_enum(enum_class, value) helper in base.py
• Added UNKNOWN member to OffReason and AlarmReason
• Updated parsing to use parse_enum for these enums

Affected devices:

• DcDcConverter, SmartBatteryProtect, OrionXS (OffReason)
• BatteryMonitor, SmartBatteryProtect, DcEnergyMeter (AlarmReason)

Testing

All existing tests pass. The change is backward-compatible—known enum values behave identically, only unknown values now return UNKNOWN instead of raising an exception.