fix: Consider RPC channel health based on MQTT session

[allenporter]

Use the MQTT session for the health rather than the "subscription". It is straight foward to add a topic when sending an RPC so we assume we can do that on demand when sending the RPC.

In practice, we don't actually use this for any health checking that is meaningful. However, it seems worth making consistent with our stance on this.

[Copilot]

Pull request overview

This PR modifies the MQTT connection health check for RPC channels to rely solely on the MQTT session state rather than subscription status. The change aligns with the architecture where subscriptions are created on-demand when sending RPC commands.

Key changes:

• Removed subscription status check (_mqtt_unsub is not None) from is_mqtt_connected property
• Updated test assertions to reflect that MQTT can be considered connected without an active subscription

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
roborock/devices/v1_channel.py	Modified is_mqtt_connected property to check only the MQTT channel connection state, removing the subscription status requirement
tests/devices/test_v1_channel.py	Updated test assertion to expect MQTT connection to be considered healthy when the session is connected, even without an active subscription

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