VantageDmxDaliGateway.mode has no default value, crashing config fetch when Mode is absent

[sjl104]

Checklist

• This issue only contains 1 issue (if you have multiple issues, open one issue for each issue).
• This issue is not a duplicate issue of currently previous issues..

Describe the issue

aiovantage/_objects/vantage_dmx_dali_gateway.py defines mode with no default:

@dataclass(kw_only=True)
class VantageDmxDaliGateway(StationObject):
    ip_address: str = field(metadata={"name": "IPAddress"})
    mode: str | None

When a controller's XML config for a Vantage.DmxDaliGateway object doesn't include a Mode element (mine didn't, for the gateway backing a Cinema RGB light), there's no value to assign to mode and the entire initial config fetch crashes — taking down the whole integration, not just that one object.

Fix: give it a default.

mode: str | None = None

Already tested working in my own environment (patched locally in site-packages) — the integration now loads cleanly and controls devices correctly. Happy to open a PR with this one-line change.

Reproduction steps

1. Set up the Vantage InFusion integration against a controller with a Vantage.DmxDaliGateway station configured without a Mode element in its config.
2. Start/restart Home Assistant.
3. Observe the integration fails to set up, with the crash traceback pointing at VantageDmxDaliGateway field parsing.