Add explicit @classmethod decorators to Pydantic validator methods

[Copilot]

Validator methods using cls parameter lacked explicit @classmethod decorators, violating Python naming conventions that methods with cls should be classmethods.

Changes

• Updated unified_validator decorator to extract the underlying function from @classmethod wrappers when present
• Added @classmethod decorators to all validator methods: ensure_keys, validate_addr, validate_comm_error_ctr, validate_ventilation_fields, extract_sensor_values, set_optional_enum
• Added documentation explaining decorator stacking order and usage pattern

Example

Before:

@unified_validator()
def validate_addr(cls, values: dict[str, dict | str | int]) -> dict[str, dict | str | int]:
    values["Addr"] = extract_val(values.get("Addr", {}))
    return values

After:

@unified_validator()
@classmethod
def validate_addr(cls, values: dict[str, dict | str | int]) -> dict[str, dict | str | int]:
    values["Addr"] = extract_val(values.get("Addr", {}))
    return values

The validators receive the class (not instance) from Pydantic's @root_validator/@model_validator, making @classmethod semantically correct.

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